The thermal condensation of 2,4-diamino 5-bromo-6-hydroxypyrimidine with 2-amino-3-mercaptopropylaniline derivatives (6)    and 2-amino-3-mercaptopropymethyllaniline derivatives (9) resulted in 2,4-diamino-6-substituted anilinomethlyl 6,7-dihydro-8H-pyrimido[4,5-b][1,4] thiazine derivatives (13, R= H) and 2,4-diamino-6-substituted methylanilinomethlyl 6,7-dihydro-8-methyl-pyrimido[4,5-b][1,4] thiazine derivatives (13, R= CH₃) respectively. The synthesized compounds were screened for antimicrobial activity and it was found that the screened compounds have antimicrobial and antifungal activity comparable to chloramphenicol and terbinafine.