A polypeptide (p40) of approximately 40,000 molecular weight was isolated from herpes simplex virus type 1 and 2 nucleocapsids by gel filtration and ion exchange chromatography. This protein appears to be the same as protein 22a described previously (Gibson and Roizman, J. Virol. 10:1044--1052, 1972). Competition immunoassays were developed by using purified p40 and antisera prepared in guinea pigs. The assays indicated that the p40's from herpes simplex virus types 1 and 2 possess both type-specific and cross-reactive antigenic determinants. Antibodies to the p40 cross-reactive determinant reacted with antigens in simian herpes virus SA8-infected cells, but not with antigens induced by pseudorabies virus. Preliminary results indicated that a radioimmunoprecipitation test can be used to detect type-specific herpes simplex virus p40 antibodies in human sera.