The present studies were undertaken to clarify the difference of the immunogenicity evoked in rabbits immunized with a low-molecular chemical antigen (TNBS) alone, by four administration routes. Rabbits were divided into four experimental groups: TNBS sc group, i.v. group, im group and ip group (3 times/week, 10 times in total), and immune responses to TNBS were estimated by the assay systems of humoral immunoreactions (HA.PCA.Arthus) and cellular immunoreaction (DTH). The highest evoking of the immunogenicity to TNBS was noted in the sc group in the case of humoral immune response, whereas the lowest evoking in the i.v. group. The intermediate degree of the immunogenicity was elicited in the im and ip groups. On the other hand, only the sc group displayed positive cellular immune response (DTH), while such immune response was never recognized in the i.v., im and ip groups. These results were indicated that the immunogenicity to the low-molecular antigen (TNBS) was evoked exceedingly in rabbits by sc sensitization route from view of humoral and cellular immune responses.