Heparin might possess anti-thrombotic properties other than anticoagulation. The aim of the present study was to test the effects of a low-molecular weight heparin, parnaparin, on adhesive molecule-mediated platelet-polymorphonuclear (PMN) leukocyte interactions and on PMN function. Platelets and PMN were isolated from citrated blood from healthy subjects. Pre-activated platelets incubated with PMN under dynamic conditions formed mixed cell aggregates. In previous experiments PMN were stimulated in vitro by purified P-selectin or formyl-methionyl-leucyl-phenylalanine (fMLP). Dual color flow cytometry was used to detect the formation of platelet-PMN mixed cell aggregates, and PMN activation was tested for by measuring L-selectin shedding, tissue factor expression and PMN degranulation. The effect of parnaparin was compared to that of unfractionated heparin. Parnaparin, at a concentration of 0.3-0.8 IUaXa/mL, inhibited the formation of mixed cell aggregates (48.8+/-9.7% of total PMN population) by up to 60% in a concentration-dependent manner, while heparin inhibited aggregation up to 40%. Parnaparin,(0.3-0.8 IUaXa/mL), prevented L-selectin shedding from PMN, which was induced by purified P-selectin (5 mg/mL) or fMLP (0.5 mmol/L) by 65% and 67%, respectively. Inhibition was independent of incubation time (5-20 min). Parnaparin (0.8 IUaXa/mL) also inhibited tissue factor expression on PMN (% of positive cells), which was induced by P-selectin or fMLP (185+/-10 and 241+/-80% of basal value, respectively). Parnaparin protected PMN from degranulation after challenge with either stimulus (> 95% inhibition). All the effects of parnaparin were observed with heparin at similar concentrations, although to a lesser extent and were often not significantly different from events in controls. In conclusion, the process of depolymerization of heparin to obtain low molecular weight parnaparin resulted in an increased, anticoagulant-independent effect on PMN function. Thus, the overall anti-thrombotic properties of parnaparin may be partly due to a leukocyte-mediated anti-inflammatory effect.