The purpose of this work was to determine if the fine specificity of T cells differed between mice immunized with an antigen in a T helper 1 (Th1) cytokine-dominated environment as compared with a T helper 2 (Th2) cytokine-dominated environment. It was found that splenic T cells from mice immunized with mycobacterial heat-shock protein (hsp 65) and interleukin-12 (IL-12) produced less interleukin-4 (IL-4) and more interferon-gamma (IFN-gamma) in response to stimulation with hsp 65 in vitro than did T cells from mice immunized with hsp 65 alone. The T-cell proliferative response to hsp 65 did not differ between the two groups of mice, although the responses were higher than those of T cells from non-immunized mice. Strikingly, T cells from mice given hsp 65 and IL-12 gave significantly higher responses to six peptides (corresponding to the sequence of hsp 65) to which T cells from mice immunized with hsp 65 alone did not respond. It is considered that different epitopes are presented to T cells (possibly owing to changes in antigen processing) if the environment is shifted, by IL-12, from Th2 towards Th1 cytokines.