Infectious bovine keratoconjunctivitis (IBKC) is a common infectious disease of cattle in Queensland, but the aetiology of the condition is not understood. A study was undertaken in which the microbial flora of the disease eyes was compared with that of apparently normal eyes, and experimental disease was established with some of the micro-organisms considered to be involved in the disease. The conjunctiva! cytology and pathology and the serological response was inves­ tigated in natural and experimental infections. An examination of the normal bacterial flora of the eye was made in 408 clinically normal cattle from eight herds and used as a basis to interpret the significance of the bacterial flora of 208 cattle with IBKC from 13 outbreaks of the disease in four widely scattered areas of south-east Queensland. The following bacterial species were isolated from normal and affected bovine eyes: Moraxella bovis, M.liguefaciens, M.nonliguefaciens, an unidentified species of Moraxella, Neisseria catarrhalis (both haemolytic and non-haemolytic varieties), an unidentified species of Neisseria, Acinetobacter metalcaligines, unidentified Gram-negative rods, Staphylococcus aureus, Micrococcus species, Streptococcus faecalis, corynebacteria and Bacillus species. M.bovis and N.catarrhalis (both haemolytic and non-haemolytic varieties) were the only two species to increase in incidence in IBKC in comparison to the normal eye. M.bovis was also shown to increase in incidence in summer in normal cattle without the development of any clinical disease. The occurrence of the disease in summer then was probably due to the seasonal increase of M.bovis induced by unknown environmental factors, but the onset of the disease required agents or factors in addition to this bacterium. Serological evidence of a current M.bovis infection was observed in three outbreaks. Conjunctiva! cytology in these herds was characterised by a predominance of polymorphonuclear leucocytes and intracellular Moraxella type bacteria. An IBKC like disease was readily induced with recent isolates of M.bovis. The conjunctivitis in such cases was catarrhal in type. In some outbreaks of IBKC few bacteria were isolated and M.bovis was completely absent indicating that agents in addition to M.bovis were involved with IBKC in Queensland. From such outbreaks a total of 17 adenoviruses were isolated. All these viruses required from two to eight blind passages in bovine testis cell cultures before detectable cytopathic effects (CPE) developed. The isolates were considered to be adenoviruses on the basis of their typical adenovirus CPE, resistance to ether and chloroform, probable deoxyribosenucleic acid content, heat resistance, common complement-fixing anti­ gens with a known human adenovirus, particle morphology and intranuclear location in infected cell sections examined with an electron microscope. The isolates were divisible into two serotypes, KC1 and KC2, on the basis of their CPE and type-specific neutralising antigens. Sixteen of the isolates were of the KC1 serotype. A poor and transient antibody increase to the KC1 serotype was demonstrated in three of the outbreaks, in two of which adenovirus isolation was attempted and was successful. The conjunctival cytology in these outbreaks was characterised by a predominance of mononuclear inflammatory cells, occasional large basophilic intranuclear inclusions of adenovirus type and often an absence of intracelluiar bacteria. Conjunctival pathology was characterised by a marked lymphoid hyperplasia in contrast to the catarrhal type of inflammation observed with M.bovis. It thus appears that at least two aetiological agents, M.bovis and adenoviruses, are involved with the IBKC complex in Queensland, the latter often in association with secondary bacterial infection. The two diseases seen in the field could not be distinguished from the clinical lesions observed but could be distinguished on cultural, serological and patho­logical evidence. The role of N.catarrhalis was not determined with certainty but it appeared to act as a secondary invader in both M.bovis and adenovirus infections and not as a primary pathogen.