Small colonic adenocarcinomas can be found in focal areas within benign tumors (adenomas), strongly suggesting an adenoma-to-carcinoma sequence. The induction of plasminogen activator (PA) secretion by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) has been used to order histologically distinct classes of human colonic adenomas in primary culture into a sequence from the most benign to the most advanced premalignant state. This ordering is based on the observation that each of five carcinomas examined in an earlier study by E. A. Friedman (Cancer Res., 41: 4588-4599, 1981) and each of seven carcinomas tested in this study released PA in response to TPA, inducing easily scored morphological alterations. Benign tumors either resembled carcinomas in their response to TPA or exhibited no morphological changes. The most benign adenomas by histopathology criteria were the small pure tubular adenomas without dysplasia. Six of seven of these adenomas did not secrete PA in response to TPA. We concluded that malignant cells had acquired the ability to respond to TPA by PA secretion, while tubular adenoma cells were not an advanced enough preneoplastic stage to so respond. TPA treatment of two cultured villous adenomas, one with infiltrating carcinoma and one with focus of moderately dysplastic cells, in the presence of low serum to decrease the plasmin concentration, demonstrated that only a subpopulation of cells secreted PA. Local areas of the monolayer were morphologically altered by the protease, forming clusters of cells loosely attached to the dish. The presence of such subpopulations within cultured adenomas was demonstrated by screening an additional five villous adenomas, 15 villotubular adenomas, and 11 tubular adenomas. The presence of dysplastic cells in 23 of 24 cases correlated with PA secretion. A subpopulation of villous cells, in the absence of dysplastic cells in each of three cases, also secreted PA. We conclude that, during tumor evolution, this villous subpopulation is the first preneoplastic cell type to acquire responsiveness to TPA by PA secretion. This property is maintained as the cells further evolve through premalignant dysplastic stages to carcinoma.