The aim of the experiments was to assess the response to topical administration of selected live strains of lactobacilli of the cells responsible for the resistance of bovine endometrium. Experimental cows (n = 8) at 8 to 12 days of the estrous received one intrauterine dose of 20 ml of a suspension of lactobacilli in 1% glucose solution. Group I (n = 4) was treated with the strain Lactobacillus spp. G 013 (5.5 x 10(8) CFU/ml) and Group II (n = 4) with the strain Lactobacillus casei CCM 1753 (1.2 x 10(8) CFU/ml). Control cows (n = 4) received 20 ml of 1% glucose solution. Samples of endometrial tissue were obtained by biopsy or from slaughtered cows on post-treatment days 5 or 6 and/or 10 or 12. Colonization of the uterine cavity with lactobacilli for up to 12 days was confirmed by bacteriological examination and scanning electron microscopy. Highly significant increases (P < 0.01) were found in numbers of all cell types under study. The pronounced cellular infiltration of endometrium was mostly due to the accumulation of mononuclear cells, particularly lymphocytes forming often indistinctly demarcated lymphoid nodules. Also marked was the infiltration of mast cells and macrophages. The cellular infiltration of endometrium persisted still on post-treatment day 12. No signs of alteration of epithelial cells were observed. No principal differences in the effects on endometrium were found between the two lactobacilli strains. The proved stimulatory effect of lactobacilli on endometrial cell defense mechanisms demonstrated in our experiments and inhibitory effects of the former on the growth of pathogenic microorganisms are promising for practical application in the prevention and alternative therapy of bovine endometritis.