Retinoic acid, the acid form of vitamin A, was found to have an inhibitory effect on the proliferation of L-929 mouse cells. Cultures treated with retinoic acid (5.0 μg/ml) were shown to cease proliferation at cell densities corresponding to confluent monolayers (10.0±1.0´104 cells/cm2). Control cultures, however, continued to proliferate and consistently reach densities two to four times higher than those of treated cultures. Viability was determined by trypan blue exclusion, and the results (80–90% viable) excluded cytotoxicity as an explanation of decreased proliferation. Replenishing the medium on confluent retinoic acid-treated cultures failed to stimulate further proliferation, while control cells continued to grow exponentially with each medium change. Therefore, the cessation of cell proliferation at confluence did not result from medium depletion. Studies of cell growth after seeding at relatively low cell densities have indicated that retinoic acid-treated cultures had greater nutritional requirements than did control cultures. Cell-cloning experiments have shown that DNA synthesis was not blocked, since clones formed by cells seeded with retinoic acid contained an average of 27 cells after 9 days of incubation (indicating between four and five cell divisions). However, clones developing from treated cells had fewer cells per clone (27 vs. 54) and were less dense than control clones. These data suggested the restoration of contact inhibition (topoinhibition) to L-929 cells treated with retinoic acid. © 1977 Oxford University Press.