Sequential serum samples from 55 homosexual men with primary HIV infection were tested for IgM anti-HIV. An early IgM response was found in 27 out of 55 (49%). In five cases IgM anti-HIV was detected 1–3 1/4 months prior to IgG anti-HIV seroconversion, as detected by a commercially available ELISA, but in no case was IgM detected prior to IgG anti-HIV seroconversion, as detected by the more sensitive GACRIA (IgG antibody captive radio-immunoassay, see Subjects and methods) and immunoblot assays. In 22 out of 23 men (96%) the primary IgM response did not persist beyond 3 months.HIV antigenaemia was found before HIV antibody seroconversion in 6 out of 55 (11%) and concomitant with HIV antibody seroconversion in 8 out of 55 (15%) subjects.A 'flu-like' illness that might be ascribed to primary HIV infection was found in 37 out of 50 men (74%). A blood sample was taken from 11 men during or within 2 weeks of the illness: no serological markers of HIV infection were detected in four subjects, HIV antigen, IgM and IgG anti-HIV were detected in another four, HIV antigen was the only marker of HIV infection in two subjects, and in one subject, IgM and IgG anti-HIV were detected but not HIV antigen.These results indicate that no conclusive value can be attached to a negative IgM test in suspected primary HIV infection, and that any role for IgM anti-HIV testing in blood donor screening is highly questionable.