Bovine haptoglobin, which has been recognized as an acute phase protein following tissue injury and inflammation, was detected as a 33 k + 20 k Dalton fraction in the sera from calves transported by road for 2 days. The sera also possessed suppressive activity on lymphocyte proliferative response to concanavalin A. A significant correlation (r = 0.57, P < 0.05) was observed between haptoglobin concentrations and lymphocyte suppression in the sera. Furthermore, the haptoglobin fraction obtained from acute phase sera exerted dose-dependent suppression on lymphocyte blastogenesis. These circumstantial data suggest the possible involvement of bovine haptoglobin, at least in part, as an immunomodulator in serum suppression of lymphocyte blastogenesis in transported calves.