During serial subcultures 50 micrograms per ml gentamicin and penicillin (100 U per ml)-streptomycin (100 micrograms per ml) depressed cell growth signficantly 2 weeks after the addition of the antibiotics; gentamicin, but not penicillin-streptomycin, stimulated cell growth before it became inhibitory. Removal of the antibiotics resulted in the cell yield returning to normal. The results show that these antibiotics can be harmful to cells even at concentrations thought to be safe.