Ilmofosine is a cytostatic/cytotoxic thioether phospholipid derivative. The in vivo anti-tumour activity of this compound was investigated in a methylcholanthrene (MethA)-induced fibrosarcoma and in the 3Lewis-lung carcinoma systems, respectively. Ilmofosine showed antineoplastic and antimetastatic properties at oral doses ranging from 0.625 to 40 mg/kg/day. Combination of Ilmofosine (p.o.) together with either cyclophosphamide (p.o.) or cis-DDP (i.v.) resulted in synergistic effects in vivo. These results demonstrate the in vivo antitumour activity of Ilmofosine in two tumour systems. The data indicate that direct cytostatic/cytotoxic effects of Ilmofosine are mainly responsible for its antitumour activity in vivo and which are increased by other cytotoxics.