Abstract. Retinoblastoma (RB) is the most common malignant intraocular tumor in early childhood. Imminent chemotherapy resistance diminishes the clinical-therapeutic options and emphasizes the necessity for new therapeutic approaches. The present study aimed at characterizing and comparing etoposide and cisplatin-resistant human RB cell lines with regard to changes in proliferation and apoptosis levels, anchorage independent growth behavior in vitro as well as tumor formation capacity in vivo. The proliferation rates were significantly increased in the etoposide-resistant RB cell lines Y-79, WERI-Rb1 and RB-355 reflecting significantly higher growth kinetics compared to the parental controls. In line with these findings in in vivo chicken chorioallantoic (CAM) assays, etoposide-resistant cell lines generated significantly increased numbers of tumors with higher tumor weights compared to their parental counterparts. In contrast to etoposide, the cisplatin-resistant RB cell lines Y-79, WERIRb1 and RB-355 displayed significantly increased apoptosis rates and reduced proliferation rates resulting in significantly decreased growth kinetics. Tumor formation capacity of cisplatin-resistant cell lines did not significantly change, and in comparison with parental controls cisplatin-resistant Y-79 cells displayed significantly reduced tumor weight. Soft agarose assays indicated that anchorage-independent growth of all chemotherapy-resistant cell lines analyzed was significantly decreased. Summarizing, one can state that etoposide-resistant RB cells behave more aggressively than the tumor cells of origin and potentially represent a risk factor for local relapse, while cisplatin-resistant cells show a significantly decreased tumorigenic potential.