The effect of prostaglandins PGE1, PGE2, PGF1 alpha and PGF2 alpha was investigated on the haemolysis of pig erythrocytes induced with aspirin and hypotonic (0.119 M) NaCl solution. An inhibiting effect was observed of low concentrations (2 X 10(-5) M, 2 X 10(-4) M and 2 X 10(-3) M) of aspirin on haemolysis induced with hypotonic NaCl solution, while in a concentration of 2 X 10(-2) M aspirin itself caused haemolysis which amounted to 93% of the haemolysis induced with 0.041 M NaCl solution. No differences were observed in the degree of haemolysis inhibition in relation to the time of incubation of erythrocytes with aspirin. Aspirin concentrations from 0.035 M to 0.280 M caused slight haemolysis (9-15% of the haemolysis induced with water), the 0.560 M solution caused haemolysis corresponding to 85% of the water-induced haemolysis. None of the studied prostaglandins used in concentrations of 0.4 X 10(-3) M, 0.4 X 10(-4) M and 0.4 X 10(-5) M had any significant effect on aspirin-induced haemolysis. PGE1 and PGE2 in concentrations of 0.4 X 10(-3) M, 0.4 X 10(-4) M and 0.4 X 10(-5) M inhibited haemolysis induced with 0.119 M sodium chloride solution, and the degree of haemolysis inhibition was from 8% to 35%. Prostaglandins PGF1 alpha and PGF2 alpha in the same concentrations had no protective effect.