In a previous paper (Allen (1)) some methods have been described for measuring gas production by micro-organisms. The objects of the work described here were (1) to measure the gas production (if any) by pure strains of lactic acid streptococci and by starters growing in milk; (2) to measure the total gas evolved by Bact. coli in the early stages of growth in different media, particularly milk; (3) to find the effect on gas production by Bact. coli of growing Str. lactis in the same medium with it and to measure the proportion of Str. lactis relative to Bact. coli required to inhibit gas production completely—information which has a bearing on the defects due to coliforms growing in the early stages of cheese making; (4) to find the influence on gas production by Bact. coli, with and without the concomitant growth of Str. lactis, of subjecting it to anaerobic conditions from the outset; and (5) to measure the effect due to the hydrogen alone (by absorbing the carbon dioxide) on the gas produced by Bact. coli under different conditions.