The alkaloidal fraction of Boerhaavia diffusa was studied for its effect on cellular and humoral functions in mice. Oral administration of the fraction (25-100 mg/kg) significantly inhibited SRBC-induced delayed hypersensitivity reactions in mice. However, the inhibition was observed only during post-immunisation drug treatment, while no effect during pre-immunisation drug treatment was observed. A significant dose-related increase in antibody titre was observed during pre- and post-immunisation treatment. The alkaloidal fraction failed to show any blastogenic responsiveness of murine splenocytes to Concanvalin A (Con A) and lipopolysaccharide (LPS). Similarly, it did not display any mitogenic activity. Thus, the present study has shown the in vivo immunostimulatory activity of B. diffusa alkaloidal fraction without an in vitro effect.