Measurements of soil enzymatic activity enable determination of the rate of regeneration of soils destroyed by fire. The aim of the study was to assess the influence of uncontrolled grass burning in spring on soil enzymatic activity. The object of the study was soil in the grassland ecosystem located in Turowola Kolonia near the town of Łęczna, Poland. Every year in spring uncontrolled grass burning is practised there. Soil samples for investigations were collected in May, July and October 2015, from depths of 0-5 and 5-10 cm, from three places: burnt soil, ecotone and area without fire. The activity of the following enzymes was measured in the samples: dehydrogenases, phosphatases, urease and protease. Grass burning had negative influence on the biological state of the soil under study, which was manifested by reduced activity of the enzymes catalysing the most important processes of transformation of organic matter. The investigations conducted 6 months after grass burning revealed that both in the ecotone and in the soil with burnt surface the enzymatic activity was similar to the activity in the control soil sample. This observation shows that soil with burnt surface achieves the state of relative biological balance, which is necessary for normal functioning of a grassland ecosystem.