Immunomodulatory activity of aqueous extract of Achillea wilhelmsii (25, 50 and 100 mg/kg body weight for 5 days) was evaluated on body weight, relative organ weight, delayed type of hypersensitivity (DTH) response and haemagglutination titre (HT) in female Swiss albino mice. No significant body weight gain differences were recorded in various groups of animals. Significant increase in relative organ weight of spleen at 100 mg/kg was observed. No elevation in the levels of liver function test (LFT) enzymes and kidney relative weight was observed in tested doses of the plant. The extract of A. wilhelmsii elicited a significant increase in the DTH response at the dose of 100 mg/kg. In the HT test, plant extract showed stimulatory effect in all doses, however this changes were significant at 50 mg/kg. No mortality was occurred in tested doses. Overall, A. wilhelmsii showed a stimulatory effect on both humoral and cellular immune functions in mice.