The effect of hapten density on benzylpenicilloyl (BPO)-protein conjugates upon the induction of BPO-specific IgE immune response was investigated in BALB/c and C3H mice. Bovine γ-globulin (BGG) and ovalbumin (OA) were employed as carrier proteins. In both strains of mice, moderately substituted conjugates (BPO19-BGG, and BPO-OA with an epitope density from 2 to 7.5) elicited a persistent BPO-specific IgE, while heavily substituted conjugates (BPO38-BGG) induced only a transient response. In contrast, lightly substituted conjugates (BPO3.2-, BPO9.5-BGG, and BPO1-OA) failed to produce BPO-specific IgE antibodies. There was no significant difference in carrier-specific IgE immune response among these various conjugates. These results suggest that moderately substituted hapten-protein conjugates are one of the favorable conditions for eliciting hapten-specific IgE immune response in mice.