Critical parameters of alkyl-lysophospholipid (ALP) induced destruction of freshly isolated human leukemic cells have been evaluated. The destructive activity of ALP is shown to be competitively inhibited by metabolizable lysophospholipids added to the cultures. It has also been found that destruction depends on the amount of serum present. Temperature and Ph strongly influence the cytotoxic activity of ALP. A slight decrease in temperature causes a reduction in cell death, whereas a temperature increase results in a marked potentiation. At low pH ALP cytotoxicity is inhibited. Incubation of cells with combinations of ALP and other cytotoxic drugs revealed a striking cytotoxic synergism with vinca-alkaloids, whereas corticosteroids retarded ALP induced cell destruction.