Potentially lethal damage repair or recovery (PLDR) after X-irradiation was examined in three experimental murine tumors, EMT6 sarcoma, SCCVII carcinoma and RIF1 sarcoma, by an in vivo-in vitro assay. All tumors were transplanted subcutaneously (SC) and, in the case of EMT6, also intradermally (id), in the thighs of syngeneic mice. Apparent PLDR was observed at high dose levels in the id and sc EMT6 sarcomas and in the sc SCCVII carcinoma, but not in the sc RIF1 sarcoma. In both id EMT6 and SCCVII tumors, PLDR appeared to be complete within 9 hr after X-irradiation. Dose-modifying factors due to PLDR evaluated at the surviving fraction of 0.01 were 1.34, 1.15, 1.24 and 0.97 for id EMT6, sc EMT6, SCCVII and RIF1 tumors, respectively. The amount of PLDR correlated inversely with the velocity of tumor growth in vivo, suggesting that slowly growing tumors might be proficient in PLDR. Although PLDR in id EMT6 and SCCVII tumors was evident in the dose range above 15 Gy, the capacity to recovery from PLD became smaller at lower doses; in SCCVII tumors, this phenomenon was negligible below 7.5 Gy. PLDR did not seem to be a major factor influencing the radiocurability of these murine tumors, especially at low dose levels.