Febrile neutropenia (FN) remains a potentially life-threatening complication of anticancer chemotherapy. Bacterial translocation via intestinal mucosa is a significant mechanism of FN development. Competitive inhibition of bowel colonization by pathogenic microorganisms by lactic acid bacteria could be a useful prevention of FN. The aim of the study was the evaluation of dose and safety of probiotic strain Enterococcus faecium M-74 enriched with organic selenium in patients with solid and hematological malignancies. Eleven (9 M/2F) patients were included in the study. In the first phase six patients with germ cell tumors treated by chemotherapy were included. They received prophylaxis by nonpathogenic strain E. faecium M-74 during 2 cycles of chemotherapy. The planned daily dose was 6 x 10(9) bacteria. Regarding the insufficient colonization of the gut, the dose was further increased up to 18 x 10(9) tid. After safety evaluation, five patients were included with relapse of acute leukemia. In patients with germ cell cancer, severe neutropenia G3/4 was noted in 10 of 12 cycles of chemotherapy. The febrile episode was not observed in any of the patients. The gut colonization by enterococci reaches 10(6) CFU/g stool. In 5 patients with acute leukemia during 127 days of severe neutropenia 12 febrile episodes occurred. There was not noted any febrile episode or infection provoked by the tested strain. Tolerance of therapy was excellent without significant undesirable effects. Optimal dose was assessed and safety of probiotic strain was evaluated in neutropenic patients with solid, or hematological malignancies. Based on these results we plan phase II study to evaluate the effectiveness of this strain in FN prophylaxis.