PIP: Potential adjuvants for use in immunizing women against beta hCG (beta-chain human chorionic gonadotropin) for contraception were tested in mice for their trapping effect: trapping refers to the sequestation of lymphocytes in the draining lymph node after immunization. The substances tested were Freund's complete adjuvant (FCA) as a positive control, Corynebacterium parvum, tetanus toxoid as a carrier protein for beta-hCG, hCG, beta-hcg-tetanus toxoid conjugate + C. parvum, and saline as a negative control. FCA is unsuitable for human use. Female mice were injected in the left food pad and the popliteal lymph nodes were studied histologically at 4 and 8 days. Tetanus toxoid induced enlargement and blast cell formation at 8 days. Fca caused enlargement and blast cell transformation at 4 days and marked enlargement and formation of germinal centers at 8 days. C. Parvum evoked a similar response to FCA, with slight enlargement of the right lymph nodes at Day 8. hCG brought no response except for slight enlargement at 8 days. The combined beta-hCG-tetanus toxoid + C. parvum elicited the greatest response, enlargement, hypercellular paracortex, blast cell transformation at 4 days and numerous germinal centers at 8 days, with enlargement of the contralateral nodes also. Since, neither tetanus toxoid nor beta-hCG causes trapping, the use of C. parvum as an adjuvant is suggested to enhance women's immune response to beta-hCG.