Rhododendron species, having been used in traditional medicine for the treatment of inflammation, pain, cold, asthma, skin andgastro-intestinal disease, are distributed widely around the world. Mad honey obtained from the nectar of common rhododendron(Rhododendron ponticum L.), which is distributed throughout Black Sea region of Turkey, contains grayanotoxins, which are toxicditerpenes. The grayanotoxins, mostly grayanotoxins I and III, which are present in leaves, flowers and nectar responsible for toxicityof mad honey. The aim of this study was to investigate in vitro cytotoxic effects of the extract on prostate carcinoma cell lines. Duringthe flowering period of common rhododendrons gathered from the Altinordu District of Ordu and dried under suitable conditions,extracted with distilled water and lyophilized. The cytotoxic activity of the extract of common rhododendron against human prostatecarcinoma (DU145) and human prostate adenocarcinoma (PC3) cell lines by using the MTT and Neutral Red assay was evaluated. Itwas determined that the extract of common rhododendron had a dose-dependent cytotoxic effect. IC50 of the extract was found tobe 283.3 and 169.9 µg/mL in MTT assay, and 307.6 and 346.0 µg/mL in Neutral Red assay for DU145 and PC3 respectively. The fact thatcommon rhododendron has cytotoxic effects on prostate carcinoma cells suggests that it may be a potential therapeutic agent foranticarcinogenic activity.