ABSTRACT: The sialic acid binding agglutinins (SAS) are purified from three different stages (proestrus [P], estrus [E], diestrus [D]) of rat uterine homogenate. All three proteins are capable of inhibiting the in vitro phytohemagglutinin (PHA)‐induced blastogenic response of lymphocytes and thymocytes as manifested by inhibition of 3H‐thymidine incorporation into cells. The lymphocytes isolated from pregnant female rats were found to be more susceptible to inhibition. In resting lymphocyte cultures (RLC), however, the P‐and E‐SAS proteins acted differently from the D‐SAS by stimulating lymphocyte proliferation in the cultures initiated from normal rat lymphocytes. At the same dose these proteins were inhibitory to the RLC, where cultured lymphocytes were isolated from the pregnant female. Reverse hemolytic plaque formation assay results also indicated functional heterogeneity between these proteins. P‐SAS induced a potent polyclonal activation of the B cell, whereas E‐ and D‐SAS did not show much activation. SAS proteins do not have any complement‐like activity.These results, corroborated by cell viability count and 51Cr uptake tests, suggest that D‐SAS has genuine lymphoblastogenesis‐suppressive properties whereas P‐ and E‐SAS‐induced suppression in the presence of PHA is probably due to an increased cell‐density‐dependent cytotoxicity.