Derivatives of quinazolin-4-one and -4-thione are potential drugs. At present, about 25 preparations based on quinazolin-4-one are registered. An almost unexplored area is that of sulfur analogues of 2-(substituted phenyl)quinazolin-4-one derivatives, some of which also showing biological activity. Therefore, attention was focused on synthesis and biological activity of a series of mostly unpublished quinazolin-4-thione derivatives. The required 2-benzamidothiobenzamides were prepared by acylation of 2-aminothiobenzamide with benzoyl chlorides and subsequent cyclisation in basic medium. A detailed study of the cyclisation has revealed that its rate depends nonlinearly on concentration of the base used. Selected compounds have been tested for antialgal, antimycotic, and antitubeculotic activity.