A study of 12-day normal and brachypod mouse limb mesenchyme in rotation culture is described. Over a 3 1/2-h period, the rate of decline of single cells was significantly greater in mutant than in normal cultures, probably because the brachypod cells exhibited a greater degree of adhesiveness. The final size of the aggregates and their cell densities were the same by 24 h of incubation. Their pattern of chondrogenesis was different. Normal aggregates contained condensations with typical histotypic cartilage by 24 h of incubation, and were entirely chondrified by 4 days in culture. The condensations in brachypod aggregates were fewer, smaller, and delayed in their chondrogenesis. Never more than 50% of the brachypod aggregate exhibited chondrogenesis. The importance of cell contact and cell density to the chondrogenic process are discussed.