The objective of the study was to determine changes in the antioxidant capacity and polyphenol content occurring during the processing of cultivated mushrooms by lactic acid fermentation carried out using a starter culture of lactic acid bacteria (LAB). The investigation material consisted of mushrooms: Agaricus bisporus and Pleurotus ostreatus. The mushrooms blanched with the addition of salt, sucrose and spices were fermented using Lactobacillus plantarum KKP 384 as a starter culture applied in the dose of 7 log cfu/g. As for the two mushroom species investigated, a product was obtained during the first week of fermentation at a room temperature, which had a pH value below 4.1, and a LAB count at a level of 9 cfu/g. The fermented mushrooms were placed in a cold room at 4 – 6 °C, there, they were stored for 7 weeks or they were pasteurized. The total polyphenol content, determined using a Folin-Ciocateu method, was 12.3 g/kg d.m. as for Agaricus bisporus, and 5.8 g/kg d.m. as for Pleurotus ostreatus (expressed as gallic acid). The antioxidant capacity values determined using ABTS* radicals were: 60.5 and 35.9 μM of Trolox/g d.m. as for Agaricus bisporus and Pleurotus ostreatus, respectively. The process of blanching the mushrooms caused the total polyhenols to decrease by 60 – 67 %, and the antioxidant capacity to decrease by 54 – 79 % . Those parameters continued to decrease during the ongoing fermentation process, however, after three weeks of cold storage, an increasing tendency was found. The antioxidant capacity values of fermented mushrooms at the end of cold storage were 22.1 and 2.8 μM of Trolox/g d.m. as for Agaricus bisporus and Pleurotus ostreatus, respectively. The LAB count in the products was ca. 8 log cfu/g. The pasteurization process of the fermented mushrooms did not decrease the antioxidant capacity level nor the polyphenol content in the samples. The antioxidant capacity was strongly correlated with the total polyphenol content (R = 0.97, p ≥ 0.01).