In vivo platelet reactivity, expressed by plasma concentration of β-thromboglobulin (βTG) and platelet factor 4 (PF4), was determined in 57 patients with bioprosthetic heart valves: 35 had well-functioning bioprostheses (WFBP), while 22 had documented malfunctioning bioprostheses (MFBP). βTG and PF4 values in patients with WFBP were not significantly different from controls, even when these determinations were repeated at monthly intervals, whereas βTG and PF4 concentration was significantly higher in patients with MFBP compared to both groups. There was a strong positive correlation between βTG and PF4 in all subjects studied. Serum lactic dehydrogenase, indirect bilirubin level and reticulocyte count were significantly higher in patients with MFBP than in those with WFBP, but no correlation was found between platelet reactivity and rate of intravascular hemolysis. Our results suggest that there is an increased platelet release in vivo of the platelet-specific proteins in patients with MFBP not related to mechanical damage, and that bioprostheses, at least in a degenerative state, may represent a potential thrombogenic f