Testis-specific lactate dehydrogenase, LDH-X, was localized spatially and temporally in the germinal epithelium of the mouse by the indirect fluorescent antibody technique. The enzyme was first detected in midpachytene primary spermatocytes and appeared to increase in concentration as spermatogenesis progressed to the spermatid. LDH-X synthesis, therefore, is initiated in the primary spermatocyte and continues at least until sometime during spermiogenesis. The uniform fluorescence of the cells containing LDH-X suggests that it is distributed throughout the cytoplasm rather than restricted to organelles such as mitochondria.