By studying various reactions of anti-rennet serum and anticasein serum serologically as well as influences of these anti-sera on the curdling of milk by rennet, the author obtained the following results. 1) With the use of either anti-rennet serum or anti-casein serum, it has been possible to induce passive anaphylaxis in guinea pigs. 2) Anti-rennet serum has inhibited the curdling of milk by rennet. 3) The time required for curdling milk by rennet has been found to be slightly lengthened when the milk is priorly treated by anti-casein serum.