Miltefosine, an alkylphosphocholine structurally related to alkyllysophospholipids showed highly selective antitumour activity against the hormone-sensitive variant of the s.c. transplantable MXT mouse mammary adenocarcinoma, the ovary-dependent MXT (M3.2), whereas it was inactive against the hormone-insensitive MXT (M3.2) OVEX variant. A dose of 32 mg/kg miltefosine p.o. daily for 5 weeks was well tolerated. Histopathological evaluation gave no signs of gastroenteral toxicity. After therapy the microarchitecture of the MXT (M3.2) tumours changed from that of a moderately differentiated adenocarcinoma to that of an anaplastic mammary carcinoma. A dose of 16 mg/kg miltefosine p.o. daily, though in effective per se, enhanced the antitumour activity of suboptimal i.p. doses of cisplatin and the hormone-like platinum analogue meso-1,2-bis(2,6-dichloro-4-hydroxyphenyl) ethylenediamine]dichloroplatinum(II). Furthermore, it was shown, that miltefosine exhibited no (anti)hormonal properties. However, the mechanism of action of miltefosine remains unclear.