Various inbred strains of rats presented differences in sensitivity to the induction of PCA reactions by mouse reaginic antibody. This was shown to be due to the presence of non-specific IgE in the normal sera of the bad recipients. Both positive and negative interaction, as measured by PCA reactions, could be observed between sera of the two different species. Inhibition of PCA reactions was seen when positive serum was diluted with normal serum from rat and mouse strains which were bad recipients, as opposed to what occured when sera from good recipients, were used to dilute. Synergism, on the other hand, was observed when mixtures of subsensitizing doses of both rat and mouse positive sera were tested. These results give further support to the suggestion that rat mast cells receptors for rat and mouse IgE are either the same, or are present, near enough each other, to produce steric hindrance.