A method is presented which allows survival and continued differentiation of male mouse germ cells for up to 12 days in culture. The system uses continuous flow perfusion at 20 degrees C in a completely chemically defined medium. To show differentiation through meiosis, mice were treated with hydroxyurea (HU) which creates a gap in the spermatogenic line. Suspensions of testicular cells of these mice, completely lacking pachytene/diplotene stages, were put into culture. After 7 or 10 days culturing, differentiation was demonstrated by the appearance of pachytene and diplotene stages and round spermatids. The velocity of the differentiation in vitro was found the same as would be expected in vivo. The number of cells which show differentiation throughout meiosis is less than would be expected in cultures of testicular cells of mice which were not treated with hydroxyurea.