The results of our experiments in two cases of ragweed dermatitis seem to cast some light on the nature of the excitants in ragweed hay fever and asthma and in the dermatitis caused by ragweed. Further, the results of attempts at passive transference in the two cases seem to bring out clearly in which cases one may expect to find reagins and in which ones they may be expected to be absent, as well as to demonstrate with which pollen fractions the reagins enter into reaction. More important than the positive findings in these cases is the fact that an exact consideration of the findings again recalls some of the still insoluble problems concerning the action and significance of reagins. Since the publications of Brown, Milford and Coca,1of Coca2and of Gay and Ketron,3it may be regarded as established that the principal and usual cause