The effects of Trypanosoma evansi on the cellular and humoral immune responses of sheep to Pasteurella haemolytica vaccine were studied. Peripheral blood lymphocytes (PBLs) from the sheep were analysed using single and double-colour indirect immunofluorescence staining and flow cytometry to monitor changes in circulating B and T cell subsets. Serum antibody responses were assayed using the enzyme-linked immunosorbent assay technique (ELISA), in addition to measuring local skin reactions at the site of vaccine administration. Results showed significant increases in circulating B cells in all sheep after the primary (p< 0.01) and secondary (p< 0.001) vaccinations although the increases were much more dramatic in the T. evansi-infected sheep. In addition, infection induced significant increases (p< 0.004) both in proportions and numbers of CD5+ B cells with more than 70% of circulating B cells expressing the CD5 antigen and showed significant differences (p< 0.01) from those of control sheep in which vaccination alone failed to induce similar increases. Also, infection resulted in significant decreases in CD5+ (p< .003), CD4+ (p< 0.03) and CD8+ (p< .03) T cell subsets in contrast to their increases in all control animals after vaccination. Moreover, there were significant suppression of both local skin reaction (p< 0.005) and serum Ig and IgG1 (p< 0.001) antibody responses to the vaccine antigen.