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OBJECTIVES To determine the amount of circulating CD4+ cells positive for intracellular p24 antigen during HIV infection, and to correlate the results with clinical, virological and therapeutic parameters. METHODS Data were obtained from 24 anti-HIV-negative subjects (controls) and 47 anti-HIV-positive patients classified according to clinical diagnosis, serum p24-antigen assay results, and antiretroviral treatment with zidovudine, using a modified flow cytometric assay for the detection of intracellular HIV p24 antigen (p24-FCA) in circulating CD4+ lymphocytes. RESULTS The proportion of CD4+ lymphocytes positive for p24-FCA correlated well with HIV infection (1.685 +/- 1.902 versus 0.160 +/- 0.152 in controls; P < 0.001) and clinical progression [Centers for Disease Control (CDC) stage II: 1.310 +/- 1.187; CDC stage III 1.145 +/- 1.442; CDC stage IVA/C2: 2.335 +/- 2.112; CDC stage IVC1: 2.066 +/- 2.420]. The percentage of CD4+ cells positive for HIV p24-FCA was inversely correlated with an absolute peripheral blood CD4+ lymphocyte count (Spearman's rank correlation = -0.324; P < 0.05). However, there was no statistically significant difference between patients in presence (n = 27; 1.938 +/- 2.095) or absence (n = 20; 1.343 +/- 1.594) of serum p24 Ag. The variable linked most strongly to the detection of intracellular p24 in anti-HIV-positive patients was zidovudine treatment: the proportion of p24-FCA-positive CD4+ lymphocytes was significantly lower (0.825 +/- 0.910) in the treated patients (n = 25) than in the untreated patients (n = 22; 2.662 +/- 2.248; P < 0.001). CONCLUSIONS Our results suggest that CD4+ p24 Ag-FCA is a rapid and easy test for the identification of the proportion of CD4+ lymphocytes with intracellular p24 Ag, and that it could be more appropriate than serum p24 Ag assay in evaluating disease progression and efficacy of antiretroviral treatment.