Recent investigations have demonstrated that the lupus erythematosus cell phenomenon is influenced, among other things, by intracellular factors. The hypothesis was formulated that leucocytes from different animal species would react with different degrees of intensity to the same L. E. serum. Leucocytes from twelve animal species were tested by counting the number of L. E. cells, "globs" (free homogeneous basophilic masses), droplets and rosettes per 1000 white blood cells and per 1000 P.M.N. after 2 hours’ incubation in the L. E. serum. Controls consisted of similar preparations using normal human serum and homologous plasma. There was a wide range of susceptibility among the various species tested. The chicken and the horse were the most susceptible species, while man fell low in the scale of susceptibility. Classical L. E. cells were produced from leucocytes of most species tested. It is felt that a highly sensitive L. E. test can be performed, using chicken, horse, guinea pig, or dog, instead of human leucocytes.