Washed platelets (10(9)/mL) derived from normal subjects, when incubated with low-density lipoprotein (LDL; 500 micrograms protein/mL) for 2 h at 37 degrees C caused the formation of platelet-modified LDL (PL-LDL). The PL-LDL demonstrated reduced cholesterol and protein levels in comparison to control LDL (incubated without platelets); it caused an increment in vitro platelet aggregation and also an elevation in mouse peritoneal macrophage cholesterol content as well as in cholesterol esterification rate. Platelets derived from patients with homozygous familial hypercholesterolemia (HFH), which demonstrated increased platelet aggregation, failed to cause a similar modification in normal LDL. Similarly, incubation of normal platelets with HFH patient-derived LDL did not cause marked LDL modification. However, in a homologous system, incubation of HFH patient-derived platelets with HFH patient-derived LDL resulted in the formation of PL-LDL similar to that produced in a normal homologous system. To investigate the effect of platelet activation on PL-LDL formation, LDL from normal subjects was incubated with medium derived from thrombin (10 U/mL)-activated normal platelets. PL-LDL formed under the latter condition was similar to that formed with nonactivated normal human platelets. Our results thus demonstrated the production of PL-LDL that was not affected by platelet activation. The formation of PL-LDL required the presence of homologous platelets and lipoprotein.