Lamb and kid rennets were prepared by extraction of dried abomasa with 6% (w/v) NaCl-2% (w/v) H3BO3 and activation of the proenzymes at pH 2·0. Each gave one zone of precipitation on casein-agar gel diffusion, enabling them to be differentiated from calf rennet and pig pepsin. After agarose gel electrophoresis, the proteinase activity of lamb rennet occurred in chymosin and pepsin bands only, whereas kid rennet contained an additional proteinase of intermediate mobility. Relative to their milk-clotting activities, lamb and kid rennets contained less pepsin and were less proteolytic on both haemoglobin at pH 1·8 and casein at pH 5·3 than calf rennet. The milk-clotting activities of lamb and kid rennets increased less with decrease in pH and were more stable to storage at both the pH value of maximum stability and lower pH values than that of calf rennet. Neither cathepsin activity nor lipolytic activity on milk fat was detected in any of the 3 rennets, but lamb rennet caused slight hydrolysis of tributyrin.