The in vivo repair of ADR-induced cell damage was investigated in the DBA3 transplantable mouse lymphoma. After a single injection of 5 or 15 mg ADR/kg body weight into DBA/2J mice, the survival fraction of clonogens showed a 2.2- to 4.4-fold decrease at 12 or 18 hours post injection and returned to pretreatment levels within 6 hours. These changes were accompanied by the appearance and disappearance of DNA crosslinks and breaks. Because cell division and/or cell loss could not explain the return of clonogens to pretreatment level, the results strongly suggest repair of ADR damage in tumor cells in situ. Such an efficient repair mechanism, responding to a high toxic dose of ADR, constitutes a therapeutically unfavorable event that may contribute to drug resistance.