Human hair follicles were cultured in serum-free media at 31 degrees C in an atmosphere containing 95% O2 and 5% CO2. Results showed that the length of the cultured hair increased time dependently for 96 h. Histological findings revealed that the hair germinative cells maintained their normal morphology throughout the 96 h culture period. DNA synthesis in the hair bulb also increased time dependently for 96 h. Autoradiographs of 3H-thymidine-labelled follicles indicated that they were localized in the germinative cells below Auber's critical line. The effects of minoxidil sulphate on DNA synthesis in this culture system were concentration dependent. Minoxidil sulphate at concentrations of 10(-10), 10(-9) and 10(-8) M significantly increased DNA synthesis compared with DNA synthesis in the control medium. Autoradiographs of the follicles cultured in 10(-10) M minoxidil sulphate showed that 3H-thymidine localized primarily in the germinative cells below Auber's critical line. These results suggest that this organ culture system may be useful for studying DNA synthesis by hair germinative cells in serum-free media.