Peroxisomes were visualized in primary cultures of mouse Leydig cells by light microscopic immunocytochemical demonstration of catalase and two enzymes of peroxisomal lipid beta-oxidation: acyl-CoA oxidase and 3-ketoacyl-CoA thiolase. Immunofluorescent peroxisomes had the form of granules, rods or short tubules located in the cytoplasm of Leydig cells. The amount of peroxisomes in the cells and the intensity of their fluorescence was similar irrespective of the antibody used. On days 2 and 4 of the culture, most Leydig cells contained very numerous, densely packed peroxisomes. On days 6 and 8, their number and fluorescence intensity gradually decreased. The parallel measurements of testosterone concentration in the culture medium revealed a sharp decline of androgen production between days 1 and 4 of the culture. The observed precedence of the decrease in testosterone production in relation to reduction of the peroxisome number suggests that a decrease in testosterone level or factors regulating androgen synthesis may influence degradation of peroxisomes.