Since mast cells have previously been shown to be potent modulators of lymphocyte function, the effect of varying concentrations of three mast cell- or basophil- as well as skin ground substance-derived glycosaminoglycans (GAG; heparin, chondroitin sulfate, hyaluronic acid) and of histamine was investigated on rat spleen cell 3H-thymidine incorporation in the absence and presence of mitogens. Modulation proved to be different for the three GAG: chondroitin sulfate was inhibitory and hyaluronic acid enhancing, while heparin exhibited a more complex pattern, with inhibition in control and phytohemagglutinin-stimulated cultures and enhancement in concanavalin A-driven cultures, in parallel to histamine. The data suggest that ground substance GAG as well as mast cell- and basophil-derived mediators in the skin can have a marked and complex modulatory effect on the function of lymphocytes.