Corynebacterium parvum and Bordetella pertussis were compared with Freund's complete adjuvant (FCA) for their abilities to potentiate the immune response to haptenic beta-human chorionic gonadotropin covalently coupled to an atoxic 54,000-molecular-weight fragment of tetanus toxin (beta-hCG-TTII). The ability of each adjuvant to enhance production of antibodies to hCG in rabbits was measured by 125I-hCG radioimmunoassay. At sera dilutions of 1:10,000, analysis of variance for the 8-week postimmunization course showed that the mean 125I-hCG binding capacities of the C. parvum group was significantly greater overall than the B. pertussis group (P = .0002) and that the FCA-treated group had the greatest binding capacity overall (P less than .018). The mean binding capacities at 1:40,000 dilution again showed the FCA-treated group to have significantly higher anti-hCG titers overall (P less than .0015), with C. parvum potentiating a greater overall antibody response than B. pertussis (P = .001). These results indicate that FCA is the most efficacious of the three tested adjuvants in potentiating antibody production to the hapten component of beta-hCG-TTII. C. parvum was also effective at promoting an anti-beta-hCG response, although not to the same degree as FCA. B. pertussis had only minimal potentiating effect compared to FCA or C. parvum.