Cell suspensions prepared from human B-cell lymphomas were analyzed for cellular DNA and cell surface immunoglobulin (SIg) by flow cytometry. In addition to SIg-bearing neoplastic B-lymphocytes, these tumors contain nonneoplastic cells. Flow cytometry permitted selective analysis of DNA in subpopulations defined by the membrane antigen SIg and also permitted analysis of SIg of subpopulations defined by DNA content. Because measurements of DNA and SIg were made simultaneously on each cell, physical separation of cells was not required. Using propidium iodide as a fluorescent DNA probe and fluorescein isothiocyanate-labeled antibodies to stain SIg, differences in cellular DNA between SIg-bearing neoplastic cells and nonneoplastic cells from the same tissues could be demonstrated. Additionally, this simultaneous correlated analysis allowed assessment of the cell cycle dependence of SIg expression. Mean fluorescein isothiocyanate (Slg) fluorescence was greater in S- than G1-phase cells, but the ratio of mean fluorescein isothiocyanate fluorescence of cells in S phase to that of cells in G1 phase varied for different tumors.