Gangliosides enhance tumor formation in experimental animals, and high circulating concentrations of gangliosides shed by tumor cells are associated with rapid progression of human neuroblastoma. We studied these shed molecules for effects on platelet function, because platelet activation may play a role in the metastatic process. Preincubation of normal platelets in patient (tumor ganglioside-containing) serum resulted in their aggregation upon exposure to a subthreshold concentration (1 microgram/mL) of collagen (up to 34% v < 10% in normal serum) and ATP release (up to 1.1 nmol/2.5 x 10(7) platelets v < 0.2 in normal serum). Because circulating shed tumor gangliosides are lipoprotein-associated, we next assessed the effects of the serum lipoprotein fraction on platelet ATP release. The patient serum lipoprotein fraction (d > 1.210) enhanced ATP release (up to 3.1 nmol ATP), whereas the same fraction of normal serum, and both patient and control lipoprotein-depleted serum fractions (d > 1.210), were inactive (< 0.2 nmol ATP released). Finally, as little as 0.5 mumol/L patient serum gangliosides (purified from the lipoprotein fraction) caused significantly greater ATP release than did normal serum gangliosides (P < .01) and caused maximal release at 50 mumol/L (up to 3.0 nmol ATP released v < or = 0.3 nmol released by platelets exposed to normal serum gangliosides). Purified total human neuroblastoma tumor gangliosides, detected in the patient serum and isolated from LA-N5 cells, were highly active; preincubation of platelets with only 5 mumol/L of these gangliosides resulted in release of 2.5 +/- 0.1 nmol ATP. Thus, neuroblastoma patient serum, the lipoprotein fraction, and, specifically, the serum gangliosides enhance platelet activation. This activity appears to reside particularly in the tumor cell gangliosides, which are shed in vivo.