In a double-blind placebo-control study the immunomodulating effect of cimetidine treatment for one week and placebo was investigated for cell-mediated immune reactions of 22 patients with herpes zoster (HZ). The mitogen induced leukocyte migration inhibition test (LMIT) and the in vitro proliferation of the patients' lymphocytes to exogenous IL-2 were used. Before any treatment, the mitogen induced leukocyte migration inhibition capacity (LMIC) of HZ patients was found to be significantly reduced (p < 0.02) as compared to healthy blood bank donors (controls). After one week, within the same treatment, the LMIC was significantly improved (p < 0.01). The patients' lymphoproliferative response to IL-2, before any treatment, was not significantly different from that of controls (p < 0.05). However, significantly higher values (p < 0.001) were found in patients tested 7 days after the disease onset as compared to those tested after 12 days. One-week cimetidine treatment significantly improved (p < 0.05) the lymphoproliferative response to IL-2 of initially low responders and had no effect on higher responder patients. In contrast to this, after one week of placebo treatment, a significant decrease in the patients' lymphoproliferative response to IL-2 could be observed as compared to patients' initial responses (p < 0.05) or to those of controls (p < 0.05). Although the number of cases is very small. The data suggest that after cimetidine treatment, as compared to placebo, healing from skin rash and pain was achieved in a significantly shorter time (p < 0.01).