Sequential sera from 48 subjects infected with human immunodeficiency virus type I (HIV-I) were examined over 36 months for the presence of neutralising antibodies, and for specific anti-gag (p24) and anti-env (gp41) antibodies to HIV-I. Results were interpreted in terms of clinical outcome during the period 1982/3 to 1985/6. HIV-I-infected subjects who remained symptom-free, by comparison with those who manifested AIDS or AIDS-related complex (ARC), had a significantly higher titre of anti-p24 antibodies throughout the 3 years, as measured by competitive enzyme-linked immunosorbent assay and radioimmunoprecipitation. The symptomless subjects also showed a trend towards an increasing neutralising antibody titre with time. There was no relation between anti-gp41 titre and clinical outcome, nor an independent relation between anti-p24 and neutralising titre. A lower or falling titre of anti-p24 antibody was associated significantly with clinical progression, up to 27 months before development of AIDS/ARC.