Little or no attention has been given to the action of male hormone on the cellular composition of peritoneal fluid. Injections of testosterone into mice increased the number of monocytes, granulocytes and large lymphocytes in the blood (Dunn, 1954). Furthermore, androgens increased haemoglobin, erythrocytes, and the volume of packed cells in normal and castrated male rats (Korenchevsky & Hall, 1945). We have shown that the cellularity of abdominal fluid is influenced by species, sex and the oestrous cycle, and differs during and after pregnancy (McGowan & Davis, 1969 a, b; Davis & McGowan, 1968). The present study attempts to determine the influence of testosterone propionate (TP) on the cytology of the peritoneal fluid of castrated male mice. We used adult male CF-1 mice and arranged them in groups of 13–16 animals. Four groups of controls consisted of normals, sham-operated mice, uninjected castrates, and castrated mice given daily injections of 0·1