To identify the specific areas of the brain that express c-yes and c-src proteins, we examined chicken brains dissected from two-week-old birds using an immune complex kinase assay and an immune blot analysis. Highest levels of both proto-oncogene proteins were found in the cerebellum, whereas other parts of the brain, including telencephalon, diencephalon, mesencephalon and spinal cord, showed three- to six-times lower levels. Relatively low levels of the two proteins were detected in pineal body and pituitary. When the cerebellum was further dissected into three layers, molecular, granular and fibrous, both the c-yes and c-src proteins were found to be concentrated in the molecular layer and, to a lesser degree, also in the granular layer. In cerebellum and in chicken embryo fibroblasts the c-yes protein was predominantly associated with the membrane fraction, and in chicken embryo fibroblasts c-yes was labeled with radioactive myristic acid. Adult cerebellum showed a two- to three-fold increase in the c-yes protein level over that detected in embryonal cerebellum. Conversely, c-src expression in the embryonic cerebellum was relatively high and it was diminished in the adult cerebellum. Differential developmental expression of c-yes and c-src proteins in cerebellum suggests that these proteins fulfill different functions or different aspects of the same function.