Gamma irradiation is one of the methods used to sterilize melanoma cells prior to coculturing them with monocyte-derived immature dendritic cells in order to develop antitumor vaccines. However, the changes taking place in tumor cells after irradiation and their interaction with dendritic cells have been scarcely analyzed. We demonstrate here for the first time that after irradiation a fraction of tumor cells present large lipid bodies, which mainly contain triglycerides that are several-fold increased as compared to viable cells as determined by staining with Oil Red O and BODIPY 493/503 and by biochemical analysis. Phosphatidyl-choline, phosphatidyl-ethanolamine and sphingomyelin are also increased in the lipid bodies of irradiated cells. Lipid bodies do not contain the melanoma-associated antigen MART-1. After coculturing immature dendritic cells with irradiated melanoma cells, tumor cells tend to form clumps to which dendritic cells adhere. Under such conditions, dendritic cells are unable to act as stimulating cells in a mixed leukocyte reaction. However, when a maturation cocktail composed of TNF-alpha, IL-6, IL-1beta and prostaglandin E2 is added to the coculture, the tumor cells clumps disaggregate, dendritic cells remain free in suspension and their ability to efficiently stimulate allogeneic lymphocytes is restored. These results help to understand the events following melanoma cell irradiation, shed light about interactions between irradiated cells and dendritic cells, and may help to develop optimized dendritic cell vaccines for cancer therapy.