Immonomodulatory effect of fruit of C fistula, a traditional medicinal plant and its synergistic antimicrobial combination with amoxicillin named Amoxy-cassia (Patent # 1371240, Government of Pakistant) studied on humoral immune system of BALB/c mice. Animal immunized with sheep RBC and treated with C. fistula fruit, amoxycassia, amoxicillin and saline Number of activated anti-SRBC producing cell in spleen calculated by haemolytic plaque assay. Antibody titer in blood was measured by haemagglutination test. Number of plaques formed by the animal treated with Amoxy-cassia, amoxicillin, C.fistula, and normal saline were 191, 86, 53, 34 per 10(5) spleen cells respectively. Haemagglutinating Antibody (HA) titer was evaluated on post-immunized day 4, 6, 8, 10. Rising antibody titer was observed in all animals but Amoxy-cassia treated mice serum had the highest HA titer through out the experiment suggesting its therapeutic usefulness. But further detailed studies of mechanisms of immunomodulation and its probable use in immuno compromised individual are still to be investigated and their possible use as an adjuvant during vaccination programs in order to reduce number of non-responder to vaccines need to be studied.