We present an in vitro bioassay protocol for the detailed examination of mitogenic influences on purified mouse Schwann cells. This involves measurements of (i) increases in Schwann cell number per culture, (ii) incorporation of radiolabeled thymidine per culture, and (iii) the proportion of total Schwann cells that exhibit a labeled nucleus by autoradiography. Using this standard protocol we show that the mitogenic effects of fetal calf serum must be distinguished from its effects on cell retention to the culture substratum, and preparations of mouse brain or pituitary fibroblast growth factor but not mouse epidermal growth factor are very potent mouse Schwann cell mitogens. These mitogenic effects of serum and fibroblast growth factor have not been reported for rat Schwann cells.