Diseases infecting silkworms cause 30-40% cocoon crop loss. The silkworm, Bombyx mori L. defense against pathogens is provided by open circulatory system through humoral and cellular response. Three mulberry varieties, Morus alba var. Rosa, M. alba var. Morittiana and M. alba var. Laevigata, were used to study the effect of host plant and bacterial contagious infection on the humoral immune of B. mori larvae. The specific activity of related enzymes in haemolymph of fifth instar larvae was estimated. Results indicated that mulberry varieties and bacterial contagious infection had significant effects on enzymes activity values in larval haemolymph. Moreover, M. alba var. laevigata was superior in the humoral immune response.