Background: Although cockroaches are mechanical vector of many pathogens, still biological transmission of pathogens via these insects is controversial. More covenant immune defense in the hemolymph of cockroaches may take this advantage to block any adaptation of pathogens regarding to biological transmission. Lectin molecules in hemolymph of insects play as mediators of nonself recognition, in the innate immune response.Methods: Initially, lectin-carbohydrate interactions in hemolymph of American cockroach, Periplaneta, americana, have been investigated and then as a model, non pathogenic soil bacterium, Streptomyces griseus was used to specify the cockroach hemolymph reaction.Results: The results showed that the hemolymph lectin activities were inhibited with GlcNAc, galactose, mannose, arabinose, and fucose but not with fructose, lactose and glucose. Also, S. griseus cells were agglutinated with the cockroach hemolymph while in the presence of fructose, mannose and galactose, the agglutinin activities were inhibited. Introducing the soil bacterium, S. griseus, into the hemolymph increased both the lectin activities as well as the number of hemocytes from an average 101 to 2688 cells per milliliter hemolymph 6 days after injection. At the same time, the number of prohemocytes in the hemolymph declined while the granular cell numbers rapidly increased.Conclusion: Generally injecting S. griseus into the cockroach body resulted in a positive correlation between an increase in hemolymph titers and enhancement of hemocyte numbers. The results tentatively support the idea that soluble and/or cellassociated Lectins may be involved in immuno-recognition in insects and block biological transmission of pathogens.