The involvement of platelets in neovascularization was investigated in the matrigel tube formation assay, an in vitro model of angiogenesis. Platelets promoted the formation of capillary-like structures (expressed as relative tube area) number- and time-dependently. Relative tube area increased from 0.98+/-0.02 (n = 8) in the presence of 6.25 x 10(4), to 3.21+/-0.12 (n=8) in the presence of 10(6) platelets/well compared to 0.54+/-0.04 (n=8) in their absence. This increase was unaffected by acetyl salicylic acid (ASA), apyrase, and hirudin. Photographs from representative experiments, showed that platelets adhered along the differentiating endothelium. Addition of alpha-thrombin (0.1-1 i.u. ml(-1)), the nitric oxide (NO) donor sodium nitroprusside (SNP; 1-100 microM) or the NO synthase inhibitor, L-NG-arginine-methylester (L-NAME, 30-300 microM) to the assay, had no effect on tube formation compared to that seen with platelets alone. Neuraminidase (0.01 i.u./10(7) platelets), which strips sialic acid residues from membrane glycoproteins, abolished the promoting effect of platelets on tube formation. The relative tube area in the presence of neuraminidase-treated platelets was 0.81+/-0.03 (n = 8), in the presence of untreated platelets 1.69+/-0.09, P<0.001 (n=8) and in the absence of platelets, 0.80+/-0.04 (n=8). The tetrapeptide Arg-Gly-Asp-Ser (RGDS; 20-200 microM) which inhibits von Willebrand factor, fibrinogen and fibronectin-mediated adhesion, had no effect on the promoting effect of platelets on tube formation. These results indicate that platelets promote angiogenesis in vitro. This effect is largely independent from activation by alpha-thrombin, is not modified by manipulating NO and prostaglandin metabolism and proceeds possibly through adhesion of the platelets to the differentiating endothelium.