Five normal and four malignant human colon epithelial cultures initiated and maintained in our laboratories as well as the standardized in vitro human adenocarcinoma cell line HT-29 were plated in multiwell plates and incubated at 37 degrees C for 72 hours with either phosphate-buffered saline solution or pentagastrin (5 micrograms/ml). Pentagastrin stimulated normal cells to increase (p less than 0.05) in number by an average of 65 percent compared with saline control cells, whereas malignant cells increased an average of 59 percent compared with control cells. There was no difference in the magnitude of trophic effect between the normal and malignant cells. Further studies are indicated to elucidate the role of gastrin in either initiating, promoting, or both, the growth of carcinoma of the colon.