Short exposures to infrared heat are generally used to facilitate tail-vein bleeding of experimental mice as an alternative to retro-orbital bleeding for the purpose of obtaining serum samples. Altered temperatures have been shown to influence immune responses in a variable manner. This study evaluates the effects of infrared heat on the immune response. After confirming the efficacy of heat exposure as measured by alterations in body temperatures, the exposed mice were evaluated for antigen-specific antibody responses and allogeneic cytotoxic T lymphocytic (CTL) responses as readouts for humoral and cellular immune responses respectively. Antigen-specific serum antibody titers to lysozyme, bovine serum albumin, ovalbumin, diphtheria toxin and rabbit IgG antigens were analyzed in infrared heat exposed and unexposed control C57BL/6 mice that were immunized with the corresponding antigen. Significant decreases in antigen-specific antibody titers were observed only when heat exposed C57BL/6 mice were immunized with lysozyme or BSA but not with other antigens tested. These alterations were not seen in heat exposed BALB/c mice. Dialyzed serum prepared from such heat exposed C57BL/6 was also found to inhibit CTL generation in vitro and inhibited IL-2 stimulated proliferation of CTLL-2 cells. These studies suggest that the procedure of infrared heat exposure prior to tail vein bleeding may influence some immune parameters although this may not be true for all strains of mice and all antigens.