The energy (enthalpy) of the multiplication of lactobacilli on various nutrient media was measured on a DAK-1-1 differential microcalorimeter. Cultivation of lactobacilli on the nutrient media studied was accompanied by a release of energy. The time dependences of heat evolution were similar in shape to the curves of the growth of the microorganism population. The conclusion was drawn that the quantity of the evolved energy depended on the type of the nutrient medium.