The platelet aggregation test, a recently developed serological method for measuring antibodies against small size (soluble) viral antigens, was used to study herpes simplex virus infections. Sera of 100 healthy persons and paired sera from 25 hospitalized patients suffering from acute HSV infection were tested by CF and PA techniques. In the serological survey the results of CF and PA were in 98% accordance. About half (13/25) of the acute HSV infections remained negative in the PA test despite definite rises in CF antibodies. Seroconversion in PA was observed only in 4 cases. The different behaviour of PA antibodies in recurrent and acute HSV infection is discussed.