MOPC 104E plasmacytoma secretes an immunoglobulin M (IgM) paraprotein which reacts specifically with dextran B-1355. We separated plasmacytoma cells into subfractions by density gradient centrifugation. The majority of cells isolated from the peritoneal washes 3 days following i.p. transplantation of tumor were in active cell cycle and had a density of 1.055 or 1.065 g/ml. Cells isolated from 9-day-old ascites were confined to the heavy-density fraction (1.085 g/ml). The majority of these cells were not in proliferative phase. Functional analysis of the fractions using rosette formation and plaque formation assays indicated that cells in active cell cycle had less surface IgM and secreted less IgM than did cells not in cell cycle. An attempt was made to establish a cultured cell line of MOPC 104E plasmacytoma. The majority of the cells of the continuously cultured cell line were in active cell cycle, had less surface IgM, and secreted less IgM. Cultured cells acquired more cell surface IgM and actively secreted IgM following their secondary colonization in 0.8% methylcellulose. These studies showed that the new line of MOPC 104E retained the properties of the original ascites tumor. An important feature of these studies is that the density of cells within a colony is a stable property and is probably not related to cell cycle. © 1983, American Association for Cancer Research. All rights reserved.