Rat egg-cylinders at the primitive streak stage were grown in modified organ culture for 2 weeks using a chemically defined medium. Differentiation of the epidermis and cartilage was comparable to that in fully serum-supplemented medium, whereas neuroblasts were very scarce. In explants treated either with bovine serum albumin or transferrin, neuroblasts were observed, whereas the addition of NGF did not improve neuroblast differentiation. On the contrary, NGF impaired growth and tissue differentiation when compared with explants grown in serum-free medium.