Interdisciplinary cooperation between basic and clinical research has resulted in the discovery and development of alkylphosphocholines, a new class of substances for the treatment of breast cancer. In contrast to most antitumor substances, the alkylphosphocholines do not attack the cell nucleus, but the cell membrane. This report presents a systematic study which, for the first time, provides a correlation between their chemical structure, antitumor efficacy and selectivity. Through an understanding of the metabolism of tumor growth inhibiting (ether)-lysolecithins, the minimal structural requirements for the antineoplastic efficacy of these substances have been obtained. This knowledge was used to identify molecular structures which are more effective and less toxic for the organism. The active principle derived from a study of (ether)-lysolecithins active as antitumor agents represents a new class of compounds: the alkylphosphocholines. As reported here, hexadecylphosphocholine is the most promising candidate of this group of compounds. It has an extremely selective action against chemically induced, autochthonous rat mammary carcinomas. No loss of activity was observed when comparing oral and intravenous administration. Particularly striking (and favorable for long-term therapy) is the fact that immunosuppression and hematotoxicity were not found at drug concentrations which lead to complete tumor remissions. Results obtained from animal experiments have been confirmed by preliminary clinical investigations.