The physiological activity of pneumococci under different physico-chemical conditions of deep cultivation in a liquid culture medium has been studied. The culture has been shown to achieve its maximum physiological activity under the conditions of neutral pHn-7.0) and the minimum content of dissolved oxygen in the medium. The addition of 5-11% of CO2 to the gas mixture has a positive effect on cultivation, the cultures thus obtained possessing high virulence and pronounced capsules.