The reaction of 5-(1-adamantyl)-4-substituted-1,2,4-triazoline-3-thione 5a,b and 10a,b with formaldehyde solution and various primary aromatic amines or 1-substituted piperazines yielded the corresponding N-Mannich bases 6a-o, 7a-g and 11a-i. The newly synthesized N-Mannich bases 6a-o, 7a-g and 11a-i were tested for in vitro inhibitory activities against a panel of Gram-positive and Gram-negative bacteria and the yeast-like pathogenic fungus Candida albicans. The compounds 6j, 6l, 6m, 7a, 7b, 7c, 7d, 7f, 11a, 11b, 11c, 11d, 11e, 11f, 11h and 11i displayed moderate to good activity against the tested Gram-positive bacteria, while compounds 7c, 11c, 11d, 11f and 11h showed potent broad spectrum antibacterial activity. None of the newly synthesized compounds were proved to possess marked activity against C. albicans.