Three calves were experimentally inoculated with bovine immunodeficiency-like virus (BIV) to examine BIV pathogenesis. Inoculated calves produced specific antibody that could be detected from 3 to 5 weeks up to 1 year postinoculation (pi). Virus was isolated from peripheral blood mononuclear cells (PBMC) 3-4 weeks pi by syncytia assay. Thereafter, the virus could be continually isolated. BIV could be isolated from monocytes but not from T cells. Likewise, monocytes could be infected with BIV in vitro. Various monocyte functions of these BIV-infected calves and age-matched uninfected calves were tested; superoxide anion release, phagocytic activity, and chemotactic responsiveness of monocytes were depressed in BIV-infected calves compared with control calves. A slight delay in the humoral immune response against mouse serum protein was also evident. During the observation period of approximately 1 year, no significant clinical symptoms could be observed. One calf, however, was killed at 15 months pi. At the time of necropsy, BIV could be isolated from PBMC as well as from cells of the spleen, liver, and lymph nodes.