International Standard lactogen, which contains 1 I. U. per 0.1 mg., was assayed by the pigeon crop response, using 3 different methods of inj., namely: shallow intrapectoral, subcut. over the pectoral muscles, and intradermal over each crop gland (micro test). The unit in each case is defined as the total amt. of hormone injected during a period of 4 days which will cause a minimum but definite stimulation of the crop glands of 50 [plus or minus] 11% of 20 common pigeons weighing 300 [plus or minus] 40 gs. The subcut. method of inj. required .1 mg. of I.S. per unit, the shallow intrapectoral method 0.125 mg. per unit, and the intradermal route required only 0.000624 mg. per unit or l/160th of the amt. required by the subcut. method. In the case of this latter method of assay, it was shown that 2. 3 and 5 fold differences in inj. vol. containing the same amt. of hormone elicited no difference in the crop gland response. The comparative relation of various other lactogenic units commonly used to the I.U. is also given.