In vitro anti-mycobacterial activities of novel 4-amino-5-alkynylpyrimidine-2(1H)-ones were investigated. 4-Amino-5-heptynylpyrimidine-2(1H)-one (3) and 4-amino-5-(2-phenylethynyl)pyrimidine-2(1H)-one (7) displayed potent in vitro activity against Mycobacterium bovis and Mycobacterium tuberculosis. Compounds 3 and 7 were also assessed for their in vivo activity in BALB/c mice infected with M. tuberculosis (H37Ra). Both compounds showed promising in vivo efficacy at a dose of 25 mg/kg for 2 weeks. Importantly, compounds 3 and 7 interacted synergistically with the front-line anti-tuberculosis drug isoniazid in vitro and in vivo. These results suggest that this class of compounds has strong anti-mycobacterial potential.