HLA class II molecules (HLA-DR, -DQ, -DP) appear to differ in their ability to serve as corecognition elements in antigen presentation to T lymphocytes. Expression of these molecules on the autologous malignant B cell lines BALM-3, -4, and -5 was studied by binding of alloantisera and by indirect immunofluorescence and immune precipitation performed with well-characterized monoclonal antibodies. The following phenotypes were identified: BALM-5, HLA-DR+, -DP+; BALM-4, HLA-DR-, -DP+; BALM-3, HLA-DR-, -DP-. When treated with phorbol acetate (TPA), all three cell lines synthesize and express both HLA-DR and -DP molecules, indicating that the structural genes for these molecules remain intact. Thus HLA-DP can be expressed with or without HLA-DR molecules. Immune precipitation studies of metabolically labeled BALM-4 cells detect HLA-DR molecules in cells from TPA-treated but not control cultures, suggesting that TPA acts by inducing the transcription and/or translation of the genes for these class II molecules. HLA-DP molecules are detected in cells from both BALM-4 cultures. Recent studies suggest that BALM-5 but not BALM-3 or BALM-4 cells are HLA-DQ positive. TPA also appears to induce expression of HLA-DQ molecules on the latter two cell lines. The unique class II phenotypes of these autologous B cell lines in the resting state therefore appear to reflect differences in their ability to execute discrete steps leading to the surface expression of individual class II molecules. Variable expression of individual class II molecules by different cell populations may affect their ability to present cell-associated antigens to T lymphocytes.