The effects of various combinations of antibacterial (ampicillin, cephadroxil, doxycycline, imipenem, trimethoprimsulfadiazine) and antineoplastic (cisplatin, epirubicin, mitoxantrone) drugs were evaluated in vitro with regard to antibacterial activity on five clinical isolates from cancer patients of respectively Escherichia coli, Staphylococcus aureus and Streptococcus faecalis. With one exception no significant effects on the bacterial growth were observed in the presence of the antitumor drug alone. In contrast, all five strains of Strept. faecalis grew better when mitoxantrone was included in a concentration of 0.1 mg/l. A synergistic action between imipenem and mitoxantrone was seen for single strains of Staph. aureus and E. coli. Furthermore, a dose-effect related inhibition by cisplatin on the growth of Strept. faecalis in the presence of sub MIC levels of trimethoprimsulfadiazine was observed. The study indicates that interaction between antibacterial and antineoplastic drugs is an erratic phenomenon, which has to be dealt with separately for each combination of drugs as well as for each bacterial strain.