The reagin response to hemocyanin (KLH) and egg albumin (EA) was studied in inbred strains (Balb/c, 129, C3H/A, C57BL/6J) and Swiss-Copacabana mice. Both antigens induced apparent and in certain experimental conditions, persistent reagin production, KLH being more effective. Al(OH)3 gel as an adjuvant was more suitable for IgE antibody production, while the Freund's complete adjuvant appeared more appropriate for IgG1 antibody response. The reaginic response to EA and Al(OH)3 in rats Wistar, Sprague-Dawley and F1 hybrids (Wistar x August) was weak and transient. The best sensitization, though relatively low as compared with mouse sensitization, was obtained in Sprague-Dawley and F1 hybrids. The results suggest, that a mouse model of sensitization is quite suitable for studying the persistent reagin response, while a rat model may be useful for investigating the early suppression of reagin response.