There is no question that the in vitro dye test of Sabin and Feldman (1948) is a very useful tool for the study of Toxoplasma infections. The results obtained in different laboratories in the United States have been, in general, consistent, as indicated by “blindfold” comparative tests. However, there are indications in reports from other countries that the test, still relatively unstandardized, may yield surprisingly low titers in cases of toxoplasmosis (Westphal, 1951). Standardization of the test has been attempted by Beverly and Beattie (1952) on the basis of the number of parasites present in the peritoneal exudates used in each particular run. In our own routine performance of the test in this Laboratory, we have encountered instances of interference or inhibition of the dye test reaction, the analysis of which has yielded information of considerable importance relative to standardization. It is the purpose of this paper to describe this phenomenon and present evidence bearing on its elucidation.