The rabbit hyperimmunization model has previously been used to evaluate candidate hypoallergenic protein ingredients. Use of the model has been expanded to include the evaluation of protein hydrolysate formulas. Each formula's immunological reactivity was determined by ELISA measurement of formula-specific rabbit antibody. Animals hyperimmunized with formulas containing extensively hydrolyzed proteins (Alimentum, Nutramigen, and Pregestimil) generated very weak immune responses (< 100 fold antibody increase). Products containing intact or partially hydrolyzed proteins (Similac, Enfamil, Good Start, Beba HA, and Nidina HA) elicited high level (> 10,000 fold increase) immune responses. Immunogenicity results were then compared to measurements of residual antigen content (by inhibition ELISA) and clinical performance. Correlation of formula immunogenicity, antigenicity and clinical performance indicates that the rabbit model is useful for screening "hypoallergenic" formulas to predict allergenic reactivity.