To compare two published methods for determining plasma viraemia in HIV-seropositive patients, with reference to a cellular viraemia assay. Three patient groups were defined according to CD4 cell count: group I, 500 × 106/I (13 patients). The two reported methodologies were applied to all fresh samples, simultaneously and on the same day. The two techniques did not differ significantly in the detection of plasma viraemia: 82.3% of group I patients and 55% of group II patients were positive, while all group III patients were negative. Cellular viraemia was positive for 96% of the overall population. These results, obtained in a network of seven French laboratories involved in clinical trials, confirm that both plasma viraemia and cellular viraemia are useful virological markers.