The invention relates to chemistry and pharmacology, in particular to synthetic biologically active substances of the pyrimidine series that have antimicrobial activity, and to the method of synthesis thereof. According to the invention there is provided a new substance with activity against gram positive bacteria, mycobacteria and fungi characterized in that it constitutes 5-(2-hydroxybenzylidene) derivatives of the pyrimidinone series with the general formula 1 where XI, X3 are selected from the group containing: H, halogen; N02; X2, X4 are selected from the group containing: H, halogen: Z is selected from the group containing: 0, NNH2, NOH, pei†iydropyrimidine-5-ytidene-2,4,6-trione, perhydropyrimidine-5-ylidene-2,4,6-trione, substituted at the nitrogen atom With alkyl, aryl or aralkyl group; Y is selected from the group containing: H, Na, Li, K.