Monoclonal antibody blocking radioimmunoassays (MAb block RIAs) which detect specific humoral responses to each of the two human herpes simplex virus (HSV) types are described. RIAs were compared with Western blot assay (WBA) in a blind study of 64 sera obtained from clinically well-documented cases of genital herpes. WBA and MAb block RIA each detected HSV-1 antibodies in 16/17 (94%) sera from confirmed HSV-1 genital infections (first episodes and recurrent infections). Detection of HSV-2 antibody in 21 sera from HSV-2 first episodes was more effective by WBA which identified homologous antibody in 19 (96%), whereas MAb block RIA detected HSV-2 antibody in 16 (76%). HSV-2 antibody was detected in 24/25 (96%) sera from recurrent HSV-2 infections by WBA and by MAb block RIA, the highest degree of concordance for both methods. In addition, the MAb block RIA may be more effective in detecting the presence of HSV-1 antibody in sera from recurrent HSV-2 cases. Prevalence of HSV-1 and HSV-2 antibody was measured by the MAb block RIAs in 3 UK human study populations which consisted of 100 children/young adolescents, 104 adult blood donors and 80 genito-urinary medicine clinic attenders.