The effect of EGF on the soft agar colony-forming ability of fresh human tumor cells was assessed in 40 specimens obtained from various types of carcinoma including those of the breast, endometrium ovary, and other sites. Cells from four established human tumor cell lines (three breast and one endometrial) were also included in this study. The results showed that addition of EGF at a concentration of 50 ng/ml resulted in a 50% higher cloning efficiency in soft agar in 40% of the samples of fresh human tumors. When cells from tumor cell lines were plated in semi-solid medium containing EGF, the number of colonies formed was at least twice as high as controls. Cells from fresh tumor biopsies were assayed for EGF receptors to determine whether the correlation between the proliferative response in EGF-supplemented semi-solid medium as compared to control could be related to the number of EGF receptors present on the cells. Specific receptors for EGF were detected by using radioiodinated EGF in early-passage cell cultures from some of the tumors tested for clonogenicity. The number of receptors ranged from 0.3 to 3.27 X 10(5) per cell. Cells from two melanoma specimens possessed less than 0.3 X 10(3) EGF receptors per cell. We found no correlation between the number of EGF receptor on a cell surface and the mitogenic effect of EGF on the same tumor cells grown in semi-solid medium.