Alkyl-lysophospholipids (ALP) are synthetic analogs of the naturally occurring 2-lysophosphatidylcholine. Some of these compounds show significant prophylactic and therapeutic activity against the growth of various allogeneic and syngeneic mouse tumors. Furthermore, they reproducibly inhibit the development of metastases of Lewis lung carcinoma in syngeneic mice. The antitumor action of ALP is mediated by an induction of cytotoxic macrophages and by a direct, selective destruction of neoplastic cells. Among other possible mechanisms, both phenomena might be based on a disturbance of the phospholipid metabolism in the tumor cell membrane, due to a lack of a 1-0-alkyl cleavage enzyme. In preliminary studies, ALP revealed only limited toxicity.