A line of human colon carcinoma cells, designated MOSER, was established which synthesized tumor-inhibitory factor (TIF) and transforming growth factor (TGF) activity. Both activities were found in serum-free conditioned medium and in cell extracts. The activities coelute on Bio-Gel P-10 in acetic acid, but can be completely separated by reverse-phase high-pressure liquid chromatography. The TIF and TGF activities were acid and heat stable and were sensitive to trypsin and dithiothreitol. MOSER cell TIF prevented the anchorage-independent growth of the more differentiated colon carcinoma cell lines tested but did not affect the less differentiated lines. Using anchorage-dependent growth conditions, the effect of TIF appeared to be noncytotoxic and partially reversible. Purified TGF stimulated the growth of normal rat kidney fibroblasts and the slow-growing CBS colon carcinoma cell line but did not stimulate MOSER cell growth. MOSER cells contain both positive (TGF) and negative (TIF) factors with relative concentrations that may be important parameters in the regulation of cell growth.