The studies have been performed on 32 adult male rats of Wistar strain, divided into three groups. Control rats have been exposed to benzene vapours, of 1200 mg/m3 concentration, 6 hours daily, for 12 weeks. In the remaining two groups, before the exposure to benzene vapours, the rats had been given, to their stomachs, sodium selenate at the dose of 1.0 and 5.0 microgram/kg (converting to selenium), for 10 consecutive days. In the blood from a tail, before the experiment and after 6 and 12 weeks of exposure to benzene, some hematological indices were determined. In control group and in rats which before the exposure had been administered selenium at the dose of 1.0 microgram/kg, lymphocytopenia, correlating with duration of exposure, and thrombocytopenia were found. In rats which before the exposure had been given selenium at the dose of 5.0 microgram/kg, lymphocytopenia did not occur and thrombocytopenia occurred as late as after 12 months of exposure. The authors think that selenium at the dose of 5.0 microgram/kg, indicates a protective effect on lymphocytic system, before benzene's toxic effects.