The present work represents an in vitro antimicrobial study of harmine, harmaline and their derivatives. These alkaloids were tested against 11 Gram-positive and 12 Gram-negative bacteria and 16 species of fungi, which included 8 dermatophytes, 6 filamentous fungi and 2 Candida species. Harmine was found to have antibacterial activity against both Gram-positive and Gram-negative bacteria. Apart from Vibrio cholerae, Salmonella schottmuelleri, Streptococcus pyogenes and S. faecalis, all the bacteria were inhibited at a concentration range of 100-500 microg/ml of harmine. Harmaline and harmol also showed antibacerial activities.