The effects of anticoagulation with citrate or hirudin on heparin effects on platelet aggregation was studied with whole blood aggregometry on blood from healthy volunteers. Platelet aggregation was initiated by collagen. The heparin effect was also studied with filtragometry where hirudin was used as the anticoagulant. In citrated blood, a mean collagen dose of 0.42 +/- 0.04 micrograms/ml resulted in an impedance change of 1.1 +/- 0.3 Ohm. Preincubation with heparin doses of 0.5, 2.5 and 5 IU/ml enhanced the impedance induced by the same dose of collagen by 2.9 +/- 1.4, 11.4 +/- 1.6 and 9.9 +/- 2.3 times, respectively (p less than 0.0001, ANOVA). In hirudinized blood a similar degree of change in impedance (1.5 +/- 0.2 Ohm) was achieved at significantly lower concentration of collagen (0.08 +/- 0.006 micrograms/ml, p less than 0.0001). Preincubation with heparin in doses of 0.5, 2.5 and 5 IU/ml increased impedance by 1.5 +/- 0.5, 3.9 +/- 1.6 and 1.3 +/- 0.5 times, respectively (p less than 0.0001, ANOVA). The dose-related increments were smaller in hirudinized blood as compared to citrated blood (p less than 0.04). With filtragometry, heparin dose-dependently shortened the aggregation time (p less than 0.0007). Compared to hirudin alone as anticoagulant, heparin in an equipotent dose in these experiments shortened aggregation time (p less than 0.05). In conclusion, heparin enhanced platelet aggregation both in calcium-chelated blood and in blood anticoagulated with hirudin. Heparin also dose-dependently increased platelet aggregation in filtragometry. Thus the heparin potentiating effect on platelet aggregation seems to be independent of extracellular ionized calcium and be operative at physiological calcium concentrations.