Growth characteristics of M1, M1neo, and M1Egr-1 cells in mass culture. (A) Growth kinetics in culture medium in the absence of IL-6. (B) Growth kinetics in culture medium in the presence of different concentrations of IL-6. Cells were cultured in the presence of varying concentrations of IL-6 for 3 days. The results are presented as the percentage of untreated M1 cells (% control). For (A) and (B), data presented are the mean of three independent determinations, with standard deviations up to 13%. Cells were seeded as indicated in Materials and Methods and viable cell numbers were determined by trypan blue dye exclusion, with counting in a hemocytometer. All experiments were initiated with nonadherent cells. In each experiment, four M1Egr-1 clones were used; all gave similar results, and data are shown only for two. Similarly, in addition to parental M1 cells, 2 M1neo clones were used. All control cell lines gave similar results. (C) Representative photo-micrographs (original magnification × 100) of M1 and M1Egr-1 cells in mass culture. Cells were seeded as indicated in Materials and Methods and cultured in the absence of IL-6 for 4 days. Photomicrographs were taken before and after washing the plates with DMEM; thus, after washing, only the cells that remained attached to the surface of the tissue culture plate are shown.