The adipose conversion of 3T3-F442A cells depends on an adipogenic factor in serum. In the presence of this factor, cells grown to confluence become spherical, greatly increase the activity of their lipogenic enzymes, and accumulate triglyceride. In the absence of the adipogenic factor, the cells grow normally, but when they reach confluence they do not become spherical, do not accumulate triglyceride, and do not undergo any increase in activity of lipogenic enzymes. In cattle there is a great deal more of the adipogenic factor in the serum before birth than in the serum of grown animals. The nature of the adipogenic factor suggests that it may play an important role in the development of adipose tissue.