Herpes simplex virus-infected target cells, sensitised with antiviral antibody can be lysed by non-immune human lymphocytes (K-cells). This antibody dependent cell mediated immunity (ADCC) is a very efficient technique for destroying foreign target cells and may play a role in maintaining the localization of recurrent herpetic infections. We have used 51Cr labelled herpes virus infected target cells to examine the K-cell activity of peripheral blood lymphocytes in patients who have either malignant reticuloendothelial diseases or who are receiving cytotoxic and steroid therapy for other reasons. K-cell activity was generally within the normal range in such patients and was surprisingly stable despite the occasional use of relatively large doses of cytotoxic agents. It is unlikely, therefore, that a defect in ADCC is responsible for the dissemination of herpes virus infections that may be seen in association with the above drugs and diseases.