A direct comparison of different HIV antigen assays is very helpful in making an informed choice, not only for the testing laboratories but also for healthcare workers in the developing world who are looking for reliable and inexpensive tests/methods in the follow-up of their treated patients. As a follow-up to the study published previously [Fransen K., Martens G., Stynen D., Goris A., Nys P., Nkengasong J., Heyndrickx L., Janssens W., van der Groen G., 1997. J. Med. Virol. 53, 31--35] where only two tests have been compared, four different commercial methods for HIV antigen determination in plasma and supernatant of cell cultures have now been evaluated on a limited sample size (88): COULTER HIV-1 p24 Antigen Assay (Coulter), (Test 1) INNOTEST HIV Antigen mAb (Innogenetics) (Test 2), Genetic Systems HIV-1 Ag EIA (Sanofi-Pasteur(1)) (Test 3) and VIDAS HIV P24 II (bioMérieux) (Test 4). Of the four tests used in this study, Test 2 was by far the most sensitive test. In a population of 88 follow-up samples from 35 different patients representing all stages of infection, the test detected confirmed p24 antigen at least once in 85.7% (30/35) of these patients, versus Test 3 in 74.3% (26/35), Test 4 in 71.4% (25/35), and Test 1 in 48.6% (17/35) of the patients. Test 2 detected confirmed p24 antigen in 84.9% of the follow-up samples, followed by Test 4 (65.9%), Test 3 (64.8%) and Test 1 (39.8%). Finally, Test 2 also proved best for detecting genetically diverse isolates.