Expression of the major inducible heat-shock protein of 68 kDa (hsp68) has been analyzed in peripheral blood mononuclear cells (PBMC) from cattle and in six Theileria annulata- and two bovine leukemia virus-transformed bovine lymphoblastoid cell lines (BoLCL). By metabolic labeling, hsp68 could be detected in PBMC and BoLCL only after heat-shock, but not under normal culture conditions. Immunoblot analysis with an hsp68 reactive monoclonal antibody similarly revealed a strong hsp68 response after heat-shock in BoLCL, and no hsp68 expression under normal culture conditions. Normally kept PBMC, however, were weakly positive with the antibody. The data are discussed with respect to the constitutive expression of hsp68 seen in several other cell lines.