The leaves of Morus alba Linn. (Family: Moraceae) commonly known as mulberry are mainly used as food for the silkworms and they are sometimes eaten as vegetable or used as cattle fodder in different parts of the world. The effect of Morus alba on the immune system was evaluated by using different experimental models such as carbon clearance test, cyclophosphamide induced neutropenia, neutrophil adhesion test, effect on serum immunoglobulins, mice lethality test and indirect haemagglutination test. Methanolic extract of Morus alba was administered orally at low dose and high dose of 100 mg/kg and 1 g/kg respectively and Ocimum sanctum (100 mg/kg, po) was used as standard drug. Morus alba extract in both doses increased the levels of serum immunoglobulins and prevented the mortality induced by bovine Pasteurella multocida in mice. It also increased the circulating antibody titre in indirect haemagglutination test. On the other hand, it showed significant increase in the phagocytic index in carbon clearance assay, a significant protection against cyclophosphamide induced neutropenia and increased the adhesion of neutrophils in the neutrophil adhesion test. Hence, it was concluded that Morus alba increases both humoral immunity and cell mediated immunity.