We have cultured mouse and human lymphocytes with earthworm (Lumbricus terrestris) coelomic fluid and measured their mitogenic responses. Normal fluid was collected from untreated worms, while induced fluid was harvested from worms injected 24 h earlier with rabbit erythrocytes. At low protein concentrations in the coelomic fluid, human and mouse lymphocytes were significantly activated, as measured by incorporation of 3HTdr. The activation index for induced fluid was approximately 2.5 times that for uninduced fluid. Separation of mouse lymphocytes into B- and T-cell populations revealed that primarily T cells were activated. Normal and induced coelomic fluid contains 12 and 28 electrophoretic bands and agglutinin titers of 32 and 1024, respectively. Addition of agglutinin inhibitors or absorption of agglutinins from coelomic fluid did not alter levels of mitogenic activity, thus the relation between earthworm agglutinins and mitogens is problematic. Techniques designed to separate and purify the agglutinins are in progress to elucidate this point.