Two groups of previously synthesized thiopurine derivatives were subjected to study its antimicrobial activity against both Gram-positive, Gram –negative bacteria, yeast and fungi. The first group of the synthesized compounds, 1-methyl-6-substituted-2, 5, 7, 8 tetrahydro-2-thio-4-oxo-pyrimido [4, 5-d] pyrimidines (2-7) and the second group, 8-substituted-3-methyl-7-hydroxy-2-thioxanthines (15-20) were used. The synthesized compounds showed various inhibitory effects against Gram-positive and Gram negative bacteria as well as fungi while Yeast showed resistance towards all of the synthesized compounds. It was clear that the inhibition activity depends up on the type of substitution.