Free native horseradish peroxidase (PO) or PO coupled to either syngeneic mouse serum albumin (PO-MSA) or to xenogeneic bovine serum albumin (PO-BSA) in sterile phosphate buffered saline (PBS), were injected repeatedly into newborn BALB/c mice. Serum antibody titres were evaluated by enzyme immunoassay on 30 and 60 of age and on d 75 and 88 after 1 or 2 booster injections respectively. The response to PO was found in all sera from neonatal immunized mice with all forms of PO, but only in control adult mice immunized by PO-BSA. Immunization with either PO-MSA or PO-BSA induced mainly IgG anti-PO antibodies of high avidity while immunization with free PO resulted in the induction of both IgM and IgG anti-PO antibodies of low avidity. A large number of hybridomas with anti-PO specificity were obtained from the spleen of mice injected with PO-MSA. The results indicate that neonatal immunization of mice with PO is an effective procedure, which is probably applicable to other proteins.