BALB/c mice were injected during neonatal life with conjugates in buffered physiological saline, prepared by coupling trinitrophenyl groups (TNP) at various densities to either syngeneic mouse serum albumin (TNP-MSA) or xenogeneic bovine serum albumin (TNP-BSA). Serum samples were obtained on days 30 and 60 after birth, on days 75 and 88 after two booster injections, and monoclonal antibodies were prepared from spleens of neonatally treated mice. The antibody titres, isotypes, and specificities were evaluated by enzyme-immunoassay. It was found that the extent of the anti-TNP immune response to TNP-MSA conjugates depends on the degree of hapten substitution, which is not the case for the anti-TNP-BSA. All the TNP-MSA conjugates induced mainly IgG and only a few IgM antibodies. These antibodies reacted essentially with the TNP group but seemed to have a higher avidity for the TNP-protein conjugate used in their induction. During the course of the immunization, decreasing quantities of TNP-MSA conjugates were needed to inhibit antibody binding. A large amount of monoclonal anti-TNP antibodies was found in hybridomas obtained after neonatal treatment either with TNP-MSA or TNP-BSA. Therefore, it appears that the anti-TNP immune response obtained after antigenic stimulation with sufficiently substituted TNP-MSA conjugates possesses all the characteristics of a normally occurring humoral immune response.