From an established cell culture line obtained from a chemically-induced rat colon carcinoma, two sublines have been selected and isolated according to their susceptibility to trypsin-mediated detachment from plastic surfaces. Subline TR, the most resistant to the detaching effect of trypsin, gave progressive tumors in most of the syngeneic rats in which it was inoculated. Subline TS, which was easily detached by trypsin, also gave tumors in the syngeneic rats, but all these tumors disappeared within 3 or 4 weeks. Both sublines gave progressive tumors when injected into nude mice. This suggests that the parent cell line is heterogeneous and contains cell variants differing in their susceptibility to trypsin-mediated detachment from substrate and their sensitivity to host factors leading to acceptance or rejection of the inoculated tumor cells.