In vitro investigations were performed to study the effect of infrared Nd:YAG laser irradiation on herpes simplex virus and the viral replication in herpes-infected Vero cell microcultures. In addition, the influence of laser irradiation on human immuncompetent cells was investigated by irradiation and incubation of herpes-infected cell cultures overlaid with leucocytes and observation of leucocyte migration under agarose after laser irradiation. There was no evidence of herpes simplex virus inactivation by laser irradiation. Irradiation of Vero cell microcultures infected with virus did not influence the development of the viral cytopathic effect. However, irradiated cultures showed an increase of about 50% in the virus yield. Only a slight indication of laser influence on immunocompetent cells was found. Cell cultures incubated with leucocytes (from both seropositive and seronegative donors) and irradiated showed an inhibitory effect with about half the virus yield seen in unirradiated controls. However, there was no relation between the energy applied and yield reduction. In addition, laser light caused no change in leucocyte migration.