The influence of the organometallic complexes titanocene dichloride, titanocene dibromide, titanocene bis(hydrogenmaleinate), and titanocene bis(p-aminothiophenolate) bis(hydrochloride) on the development of a lung adenocarcinoma and a small cell lung carcinoma, both xenografted into athymic mice, was investigated in the present study. The tumors were growing s.c., and the substances administered i.p. as fivefold injections according to a Q2D X 5 (every 2 days X 5) or a Q3D X 5 schedule. In the case of lung adenocarcinoma, titanocene complexes inhibited tumor growth by more than 50% resulting in treated/control values of 20%-50%. The suppression remained stable beyond the end of the treatment period. In the case of small cell lung carcinoma, only those Q2D X 5 schedules, which corresponded to LD10 doses, effected considerable and stable growth inhibition to less than 50% of control values. Titanocene dichloride showed the greatest activity against both human tumors, followed by the p-aminothiophenolate, and hydrogenmaleinate derivatives.