The course of infection with herpes simplex virus type 1 (HSV-1) in peritoneal macrophages, phytohemagglutinin (PHA-)stimulated (T-)lymphocytes and lipopolysaccharide (LPS-)stimulated (B-)lymphocytes of NMRI-mice was studied by means of electron microscopy. Non-stimulated as well as thioglycolate-stimulated macrophages were investigated; lymphocytes were derived both from HSV-1-sensitized and non-sensitized animals. The morphological characteristics of the abortive infection in macrophages and T-lymphocytes and of the productive infection in B-lymphocytes are described. No differences were observed between stimulated and non-stimulated cells or cells of sensitized and non-sensitized animals.