Twenty-three patients with a herpetic infection as diagnosed by a positive culture of herpes simplex virus (HSV) were studied with respect to serological responses of IgA, IgG and IgM antibodies in paired serum samples by an indirect (sandwich) enzyme linked immunosorbent assay (ELISA). Eight of the patients had a primary infection and 15 a recurrent one. In the ELISA test a detergent treated cell lysate of HSV type 1 was used as antigen. In the IgM assay all sera were pretreated with antihuman IgG with the purpose to precipitate IgG of the samples. The conjugate was a F(ab)2-fragment of antihuman-IgM. In primary infections all patients had significant titre rises of IgG and presence of high IgM titres in the convalescent serum. IgA antibodies were found in all of them, while titre rises were detected in 5/8. In recurrent infections titre rises of IgG and IgA antibodies were found in 4 and 5, respectively. Six had detectable IgM in one or both of the paired samples. The IgG titres were higher in recurrent infections than in primary, in contrast to IgM of which much higher titres were found in primary infections. It is concluded that in primary infections a conclusive serological diagnosis was established in all patients, whereas in recurrent infections this was achieved in two of three patients. The indirect ELISA method used for IgM detection was sensitive, reliable and convenient. Interfering rheumatoid factor was effectively eliminated by treatment with antihuman IgG.