Prostate cancer is the second most common cancer in elderly men worldwide and its incidence rate is rising continuously. Agents capable of inducing apoptosis in prostate cancer cells seem a promising approach to treat this malignancy. In this study we describe the synthesis of a number of novel N- and N,N'-substituted S-2,3,4,5,6-pentabromobenzylisothiouronium bromides and their activity against the human prostate adenocarcinoma PC3 cell line. All the compounds produced changes in mitochondrial transmembrane potential and cell cycle progression, showed a cytostatic effect and induced apoptosis in the tested cancer line in a concentration- and time-dependent manner. The most effective compounds ZKK-3, ZKK-9 and ZKK-13 produced, at 20 microM concentration, apoptosis in 42, 46, and 66% of the cells, respectively, after 48 h incubation. Two selected S-2,3,4,5,6-pentabromobenzylisothiouronium bromides (ZKK-3, ZKK-9) showed also a synergic proapoptotic effect with the new casein kinase II inhibitor 2-(4-methylpiperazin-1-yl)-4,5,6,7-tetrabromo-1H-benzimidazole (TBIPIP) in the PC3 cell line.