Bacteriological tests were done on a large number of different strains of Actinobacillus seminis and also, repeatedly, on the same culture or on different cultures taken periodically from the same donor animal. These tests were also applied to strains of A. seminis representing different serological types, which in turn were compared with strains of Brucella ovis. The tests as applied proved that A. seminis strains have defined, morphological, staining, cultural and biochemical properties, although they can generally be regarded as biochemically inactive. Growth was greatly enhanced on media enriched with blood or serum and also more luxuriant when incubated in a carboxophilic atmosphere. Nitrate reduction was found to be a variable characteristic, as it was more often negative, while weakly positive and negative reactions for hydrogen sulphide production were encountered with equal frequency. On the basis of their bacteriological properties, the strains representing the different serological types can be divided into 2 groups. Strains belonging to the first of these groups conform to the earlier description of A. seminis by Baynes & Simmons (1960) and are usually catalase positive and oxidase negative, while those in the second group more closely resemble Histophilus ovis described by Roberts (1956), and produce variable reactions on the catalase and oxidase tests. Although growth did occur aerobically and was more luxuriant in a carboxyophilic atmosphere in all strains, it was always much slower for strains resembling H. ovis. Similarly, the growth produced by these strains was poorer and more irregular on ordinary nutrient media and, although greatly enhanced and more regular in all strains on enriched media, it was again much slower for these strains. In all stages of development, the colonies of strains similar to H. ovis were always slower and more transparent in appearance, and tended to remain low convex and undifferentiated. Packed organisms of these strains were light yellow (lemon) in colour in contrast to strains resembling A. seminis, which had a greyish-white appearance. A. seminis and B. ovis can clearly be distinguished on their morphology, Stamp staining reaction on both semen and culture smears, colonial morphology, the delayed colony development of B. ovis and sensitivity to dyes and antibiotics.