Resistance of tumor-bearing mice to a second tumor challenge, that is, concomitant resistance, was studied using the LB tumor model. In a secondary LB tumor implant inhibited by concomitant resistance an increase in the percentage or apoptotic cells and alterations in cell cycle distribution were observed. Similar alterations were observed in LB tumor cells incubated with serum from tumor-bearing mice. The data presented in this paper suggest that apoptosis is one of the mechanisms involved in tumor dormancy due to concomitant resistance.