In this work we studied 205 children with cancer, aged 6 months to 7 years, who had been diagnosed as suffering from various types of neoplasm. In blood of these children we determined the selenium concentration and glutathione peroxidase activity by fluorometric and spectrophometric methods, respectively. The control group consisted of 128 healthy children. In all groups of children with cancer we observed a significantly lower selenium concentration and lowered glutathione peroxidase activity. We found statistical differences in selenium concentration between first or second and third, and between first and fourth or fifth stages of the disease, only in 3-7 year-old patients. Glutathione peroxidase activity was statistically depressed in the same age group between the first or second and third stages of the disease. Generally, there were no differences in the concentration of the microelement or in the glutathione peroxidase activity between children before and during treatment with cytostatics.