A preparation of herpes simplex virus type 2 (HSV-2) nucleocapsid (NC) was obtained after treatment of intracellular virus with detergents followed by centrifugation in linear sucrose density gradient. Electron microscopic analysis revealed polymorphism of HSV-2 NC population. Three main types of capsid structures were found: empty capsids, NC containing various amounts of nucleoid material, and NC with complete nucleoid. Morphological heterogeneity correlated with heterogenous sedimentation profile of the total NC preparation of labeled virus represented by light, intermediate, and heavy structures. Specific blasttranformation tests with lymphocytes from rabbits immunized with HSV-2 NC demonstrated differences in the primary and secondary cellular immune responses. The secondary cellular immune response was characterized by a more rapid and strong increase in BTT values than the primary immune response.