The ability of platelets to adsorb vinblastine has been used to treat patients with immune thrombocytopenia. It is hypothesized that the drug- platelet complex is coated with antibody, taken up by macrophages which are then destroyed by the drug. We gave 16 courses of vinblastine- platelets to six patients with immune thrombocytopenia. Only one patient responded, and therefore we examined possible reasons for the lack of benefit. Using 3H-vinblastine, the kinetics of vinblastine binding to platelets was studied in vitro. The binding of vinblastine to both human and rabbit platelets was identical with maximal binding occurring within 10 min at 600 microgram/ml vinblastine. Similarly, the plasma half-life of vinblastine in rabbits was close to that reported for man, and therefore, in vivo binding of vinblastine to platelets in rabbits was considered a suitable model for man. Homologous donor rabbit platelets were labeled with 51Cr alone, 51Cr plus vinblastine, or 3H-vinblastine and infused into recipient rabbits. Vinblastine had no effect on 51Cr survival, but all measureable vinblastine had left the platelets within 2 hr of the infusion. These observations suggest that delivery of the vinblastine to the macrophages depends on the platelets being phagtocytized before the drug leaves the platelets. This would be likely to occur only in those patients with severe immune thrombocytopenia. Further investigations into this treatment should be directed at methods to maintain the drug within the platelet.