In vitro experiments showed that 250-500 micrograms.ml-1 of the polysaccharides isolated from Medicago sativa L (MSP) increased mouse lymphocyte uptake of [3H]thymidine (TdR) induced by PHA-p (0.15%), Con A (5 micrograms.ml-1), LPS (20 micrograms.ml-1), and PWM (2 micrograms.ml-1) at maximum by 5.13 +/- 0.81, 0.75 +/- 0.08, 4.52 +/- 0.36, and 0.86 +/- 0.24 folds, respectively. After MSP ip for 5 d, the average count of mouse splenic lymphocytes in 125 mg.kg-1 x d-1 group increased from 1.4 +/- 0.4 (x 10(7))/mouse to 3.6 +/- 0.5 (x 10(7))/mouse; in 250 mg.kg-1 x d-1 dose group increased to 13.5 +/- 0.3 (x 10(7)). MSP 125 mg.kg-1 x d-1 or 250 mg.kg-1 x d-1 ip for 5 d partially or completely antagonized the effect of cyclophosphamide (200 mg.kg-1 x d-1 x 1 d) on lymphocyte decrease, the average number of lymphocytes/mouse recovered from 1.04 +/- 0.23 (x 10(7)) to 2.1 +/- 0.4 (x 10(7)) and 2.9 +/- 0.6 (x 10(7)), respectively. In addition, the lymphocyte reactivity to Con A in MSP ip 125 mg.kg-1 x d-1 x 5 d group enhanced by 60%, in 250 mg.kg-1 x d-1 x 5 d group by 156%. The abilities of PWM-induced antibody production were elevated 51% and 78%, respectively.