Germination of spores of Clostridium botulinum strains 62A, 213B and 12885A was monitored in modified peptone yeast extract broth flushed with CO2, N2, CO2 + H2 and N2 + H2. Carbon dioxide enhanced germination of spores of each of the strains. Hydrogen gas in combination with CO2 or N2 did not substantially alter germination compared to germination in CO2 or N2 alone, even though the oxidation-reduction potential of the system was lower in the systems flushed with H2.