Platelets contain, besides alpha- and delta-granules, lysosomes which store glycohydrolases able to degrade glycoproteins, glycolipids and glycosaminoglycans. While several studies have shown that alpha- and delta-granule secretion takes place "in vivo" in humans upon platelet activation, no data are available on the "in vivo" release of lysosomes. We have studied the release of platelet lysosomal contents "in vivo" in healthy volunteers at a localized site of platelet activation by measuring markers of lysosomal secretion in the blood oozing from a skin wound inflicted for the measurement of the bleeding-time. The levels of beta-N-acetylhexosaminidase (Hex) were 13.1 +/- 0.85 mU/ml in bleeding-time blood and 10.2 +/- 0.66 mU/ml in plasma (p <0.001). Hex in serum was 16.4 +/- 0.72 mU/ml. The levels of beta-galactosidase were also higher in bleeding-time blood than in plasma (0.85 +/- 0.07 mU/ml vs 0.4 +/- 0.05 mU/ml, p <0.001). In bleeding-time blood collected at one minute intervals, Hex rose progressively consistent with ongoing platelet activation and flow-cytometry showed a progressive increase of the expression of LIMP and LAMP-2, two lysosomal associated proteins. In conclusion, our data demonstrate that platelet lysosomal glycohydrolases are released "in vivo" in humans upon platelet activation.