A basic immunophenotyping panel that employed dual-color combinations of fluorescein isothiocyanate (FITC) and phycoerythrin (PE) conjugated monoclonal antibodies (mAb; FITC-CD45/PE-CD14, FITC-IgG1/PE-IgG2, FITC-CD3/PE-CD8, FITC-CD3/PE-CD4, FITC-CD3/PE-CD16+PE-CD56, and PE-CD19) was utilized in a quality assurance program to determine whether the 4 laboratories participating in a multicenter AIDS study obtained similar lymphocyte subset percentage values for T cells, B cells, NK cells, and CD4+ and CD8+ T cells. Over a 1½ year period, 78 shared peripheral blood specimens Were prepared and analyzed in each laboratory. The CD45brightCD14− percentage for each specimen was used to correct that individual's lymphocyte subset values. Interlaboratory coefficients of variation (CV) for the human immunodeficiency virus type I (HIV) seronegative (n = 38) and HIV-seropositive (n = 40) specimens using this panel were <3% for total T cells; <5% for CD4+ T cells and CD8+ T cells; ≤17% for B and NK cells; and <8% for CD4T/CD8T ratios. The 6-tube basic immunophenotyping panel has several notable features: (a) for clinical studies, it permits comprehensive evaluation of an individual's major lymphocyte subsets, i.e., T, B, NK, and CD4+ and CD8+ T cells; (b) for interlaboratory proficiency testing programs, it allows the detection of differences among laboratories in measurements of several functionally distinct cell populations; and (c) for within-sample quality assurance, it provides several quality control checks, including the lym-phosum, i.e., the sum of an individual's corrected T + B + NK values, a sum that was generally 100 ± 5% on the HIV-seronegative specimens analyzed in this study. © 1993 Wiley-Liss, INc.