In a phase I study a cytotoxic activity in the serum of a tumor patient given infusions of synthetic alkyl-lysophospholipid has been demonstrated. Serum samples collected after ALP infusions inhibited 3H-thymidine incorporation by human leukemic cells to an extent that correlated to the dose of ALP administered. Serum taken after the highest dose of ALP given (50 mg/kg body weight) led to complete cell destruction after 72 h in vitro. Whereas cells from the HL60 line were very sensitive to the serum cytotoxicity, K562 cells were much less affected. Cytotoxic activity was found to be clear from the circulation in a biphasic manner; more than 50% disappeared within 6-8 h but 20-30% was still present after 4 days.