Intrinsic phagocytosis and killing of C. albicans by equine monocytes and polymorphonuclear leucocytes (PMN) was examined during Equid Herpesvirus 1 (EHV-1) (subtypes 1 or 2) and Adenovirus infections. Monocyte function increased during EHV-1 subtype 2 and Adenovirus infection. Conversely, there was an impairment of monocyte ingestion during EHV-1 subtype 1 infection which was ascribed to virus replication in peripheral blood mononuclear cells. PMN phagocytosis was not decreased in any of the infections studied. The raised levels of haemolytic complement in animals which subsequently developed EHV-1 subtype 1 induced paresis suggested an abnormality of complement turnover. Increased levels of interferon were evident in the nasal secretions of both subtype 1 and subtype 2 infected animals but only subtype 1 virus induced measurable levels of serum interferon. No intrinsic abnormality of interferon production by monocytes or lymphocytes was found.