A series of bacterial species was examined for their sensitivity to ochratoxin A. Only gram-positive bacteria could be inhibited, generally at a pH lower than 7.0. Bacillus subtilis did not show any reduction of growth rates in presence of ochratoxin A, but had a prolonged lag phase. With Staphylococcus pyogenes var. aureus and Streptococcus faecalis, a prolonged lag phase and a reduction of the growth rates was observed. Most sensitive was Streptococcus faecalis in the exponential-growth phase. The inhibition could be diminished by changing the pH to neutral, or by addition of yeast extract, tetrahydrofolate, or MgSO4. With MgSO4 a complete abolition of the inhibitory effect was achieved, but not with CaCl2. During growth inhibition, protein and RNA synthesis were reduced simultaneously, but not DNA synthesis. Even with the very high concentration of 1 mg/ml, no lethal effect was observed.