Serum and skeletal muscle-derived extracts (MDE) were bioassayed for their ability to promote [3H]thymidine incorporation in L6 myoblasts to determine if exercise-training increases mitogenic growth factor activity. Rats were trained by climbing a 60 cm vertical grid with progressively greater weight for 7 and 28 days. Serum from 7 day trained rats and vastus MDE from 7 and 28 day trained rats, but not rectus femoris MDE, had increased mitogenic activity compared to serum and MDE from untrained rats. These data suggest that exercise may increase mitogenic growth factors in some skeletal muscles.