Patients receiving high-dose chemotherapy and autologous bone marrow transplantation acquire a platelet secretion defect. The role of chemotherapeutic agents and their metabolites in mediating this platelet defect was investigated. 1,3-Bis(2-chloroethyl)-1-nitrosourea (BCNU), but not cyclophosphamide or cis-platinum, was found to inhibit platelet aggregation in vitro in response to activation by either ADP, thrombin, or collagen. Inhibition by BCNU was dose dependent and required preincubation of platelets with BCNU. After a 60-min preincubation, 30 microM BCNU produced 50% inhibition of platelets in platelet-rich plasma. The cyclophosphamide metabolites acrolein and 4-hydroperoxycyclophosphamide also inhibited platelet aggregation in a dose-dependent manner, with a requirement for preincubation. Platelet inhibition occurred at clinically relevant concentrations of BCNU and metabolites of cyclophosphamide. The effects of acrolein were totally prevented by coincubation with the sulfhydryl-protecting agents N-acetylcysteine and 2-mercaptoethanesulfonic acid, whereas the effects of BCNU were incompletely prevented. The mechanism of platelet inhibition was investigated next by examining protein phosphorylation in response to platelet agonists. Acrolein inhibited thrombin- and phorbol ester-induced phosphorylation of a 40-kDa polypeptide and other substrates, indicating a cellular defect in protein kinase C signaling. BCNU did not interfere with protein phosphorylation, indicating preservation of initial signaling pathways. Thus, chemotherapeutic agents and their metabolites inhibit platelet function by inhibiting distinct components of the intracellular activation pathways.