During a series of experiments on chick embryos, it was decided to ascertain the effect, if any, of some of the bacterial toxins on the development of the embryos. This paper deals with the results obtained with the toxic filtrate from Corynebacterium diphtheriae. The method used was essentially a modification of Good-pasture's (1) technic in which the substance to be tested was placed on the chorio-allantoic membrane of the developing chick. The application of this method to the testing of bacterial toxins is believed to be original. Experimental. The eggs used in these experiments were partly pure bred and partly fertile market eggs of mixed breeds. The pure blood eggs included the following strains: White Leghorns (white shell), Black Minorca (white shell), Rhode Island Red (brown shell), and Buff Orpington (brown shells). In the tests, only eggs between 50 to 70 grams in weight were used because of the fact that very small eggs were either infertile or were apt to contain abnormal embryos.