A series of 6-chloromethyl-3-hydroxy-2-substituted 4H-pyran-4-one derivatives were synthesized and tested for their antimicrobial and antiviral activities. Mannich base derivatives were prepared through the reaction of substituted piperazine or piperidine derivatives on chlorokojic acid and formaline. The structures of the synthesized compounds were confirmed by IR, H-1 and C-13 NMR, ESI-MS, and elemental analysis. According to the activity studies, compounds 2-7 (MIC: 1-2 mu g/mL) were found to be highly active against Bacillus subtilis and Staphylococcus aureus, while compounds 3, 5 and 6 showed significant activity against Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae and Acinetobacter baumannii. Also, compounds 2-7 were more remarkably active against Candida albicans and Candida parapsilosis (MIC: 4-8 mu g/mL). Additionally, compound 2 was the most active one against RNA virus PI-3. (C) 2010 Elsevier Masson SAS. All rights reserved.