The objective of this study was to investigate the effects of bovine leukemia virus (BLV) infection in sheep. A prospective study of the serologic, hematologic, and histologic changes of sheep infected with BLV was conducted. Antibodies to BLV were detectable in the sheep 3 weeks after exposure to blood from an infected cow and persisted during a 120 week examination period, whereas all control sheep remained seronegative. There were no statistically significant differences between the leucocyte counts, lymphocyte counts, and lymphocyte percentages of the infected and control sheep during the first 120 weeks of this study. However, one sheep did develop a leukopenia and lymphopenia 95 weeks after it became infected and died of histologically-confirmed lymphosarcoma 10 days later. A lymphocyte colony assay was used to study the effects of BLV infection on colony formation by sheep lymphocytes in vitro. There was no significant difference in the number of lymphocyte colonies formed by BLV infected and control sheep. Nor was there a significant difference in the number of colonies formed by lymphocytes from the BLV infected sheep, when the autologous sheep serum was replaced with either pooled serum from the infected sheep or with pooled serum from the control sheep. BLV infection in aleukemic sheep does not appear to have an adverse affect on colony formation by lymphocytes in vitro.