In the present studies, we examined the in vitro and in vivo effects of ganciclovir (DHPG) and a relatively new nucleoside analogue, (S)-1-(3-hydroxy-2-phosphonylmethoxy propyl) cytosine (HPMPC), on lymphocyte responses to T cell mitogens and delayed-type hypersensitivity (DTH) responses to dinitrofluorobenzene (DNFB). Initially, responses of mouse splenic mononuclear cells and human peripheral blood mononuclear cells to PHA and con A were evaluated in vitro in the presence of each drug. Both drugs inhibited the responses to each mitogen; however, DHPG had a greater inhibitory effect on con A responses of human and mouse lymphocytes than did HPMPC. Also, spleen cells from mice treated for 7 days with DHPG responded less well to PHA stimulation than cells from untreated or HPMPC-treated mice. No effect of either drug was observed on con A responses. Treatment of mice with either drug decreased the development of DTH responses, with HPMPC having a greater inhibitory effect than DHPG. The results from the present studies suggest that both DHPG and HPMPC may have inhibitory effects on the development of certain immune functions at high dosages, but at drug concentrations that were therapeutic in animal model studies, little inhibitory effects were observed.