Piper longum fruit, used in traditional remedies as well as in the Ayurvedic system of medicine against intestinal disorders, was tested for its efficacy against experimental infection of Giardia lamblia in mice. On in vitro test, an aqueous extract of P. longum fruit powder (PF) at 250 microg/mL and its ethanol extract at 125 microg/mL showed 100% giardicidal activity. A low order activity was found in the n-butanol extract. Further fractionation in hexane and chloroform resulted in a total loss of activity. The survival of-trophozoites in mice at 900 mg/kg body weight was 11.12 in PF, 8. 54 in aqueous extract, 5.81 in ethanol extract. The antigiardial activity of PF in hexane, chloroform and n-butanol soluble fractions was comparable to the drug-untreated control (47.63). Piper longum possessed a demonstrable immunostimulatory activity, both specific and nonspecific, as evident from the standard test parameters such as haemagglutination titre (HA), plaque forming cell (PFC) counts, macrophage migration index (MMI) and phagocytic index (PI). A maximum effect was found at 225 mg/kg body weight in mice. The effect was marginally reduced at higher doses of 450 and 900 mg/kg or the lower dose of 112.5 mg/kg.