A conditioned medium (CM) from a cloned rat thymic myoid cell (IT-45R92) was used to investigate a possible relationship between the thymus and brain. CM stimulated the DNA synthesis of the cultured perinatal rat brain cells and spleen myelo-lymphoid cells but not that of the other types of cells including dorsal root ganglia, spinal cord, heart muscle, skeletal muscle, kidney, liver, skin, and thymic fibroblasts. CM appeared to stimulate glioblasts to differentiate into mature astrocytes that have a characteristic intracellular fibrous structure and a glial fibrillary acidic protein. These results indicated that thymic myoid cells release (a) 'cytokine(s)' for the astrocytes and myelo-lymphoid cells, similar to a 'lymphokine' (glia cell stimulating factor, GSF) produced by murine and human lymphocytes stimulated with mitogens.