The anticarcinogenic activity of living standardized culture of Lactobacillus delbruescii sub. bulgaricus 51-L (LB) was studied on male rats “Whistar”, treated with diethylnitrosamine (DENA). The animals were divided into the following groups: I group—controls; II group—daily LB intake dose of 2 ml/animal with the drinking water; III group—treated with DENA at a dose 130 mg/kg body weight injected intraperitoneally once per week for 8 consecutive weeks; IV group—treated simultaneously with LB and DENA. The animals were monitored during a period of 16 weeks. Their general condition, weight increase, mortality rate and manifestation of tumours were observed. After the observation period the animals were killed and the following parameters were studied: glutathione-S-transferase (G-S-T) activity in liver, macroscopic examination of the animals, histomorphological and histochemical investigation of the suspected ones for tumours and precancerogenic formations as well as liver, kidneys, spleen, lymph nodes, lungs and other (if necessary) target organs. An electronic-microscopic study of liver and morphological-immunologic investigation of spleens were carried out. A high mortality rate was established in DENA treated animals as well as cirrhotic changes of their liver which in the IV group showed a tendency for restriction. Specific G-S-T activity (calculated on the basis of soluble proteins) of liver showed a general tendency for increase in all groups at statistical significance in the group with the combined effect of DENA and LB. The induced tumour genesis mainly hepatic one affected 98% of the animals belonging to the III group and 83% of the animals of the IV group. A diagnostic of the induced benign and malignant tumours was performed. In the IV group statistically significant decrease (p < 0.01) of the number of tumours in a single animal was established versus that of the III group (1.6 versus 3.26), which is an evidence for the inhibiting effect of LB in relation to the DENA-induced cancerogenesis. Immuno-morphometric study on the spleens showed that in the IV group there was certain stimulation of the zones of B-lymphocytes at a preserved suppressive effect of DENA on the zones of T-lymphocytes. The possible mechanisms of the inhibiting effect of LB, which probably include activation of the detoxification mechanisms of G-S-T were also discussed.