Radiobiologically hypoxic fractions of 4 experimental tumors (RIF1, B16, EMT6/KU, SCCVII) of various sizes implanted subcutaneously (sc) or intradermally (id) were compared under identical conditions by using the paired survival curve assay method. The two survival curves for tumors in air-breathing and asphyxiated mice were almost parallel for EMT6/KU and SCCVII tumors, but not for RIFT and B16 tumors. Therefore, hypoxic fractions were estimated by fitting the best parallel lines to the two sets of survival data. The values for 10mm-diameter se tumors in the hind legs of syngeneic mice were 4.7% for RIFT, 4.5% for B16, 14% for EMT6/KU, and 8.5% for SCCVII. The variation of the values is quite small in spite of the variety of biological characteristics of these tumors. Histological examination revealed that EMT6/KU and B16 tumors contained large necrotic areas, while SCCVII and RIF1 had small areas of necrosis. Thus, the hypoxic fraction and the proportion of necrosis in the histological specimens were not clearly correlated. The values for 6mm and 16mm sc EMT6/KU tumors were 5.9 and 22%, respectively, while that for 6mm id tumors was 16%. The values for 5mm and 18mm sc SCCVII tumors were 0.86 and 8.4%, respectively. These results indicated that: (1) id EMT6/KU tumors had a higher hypoxic fraction than sc ones of the same size; (2) the hypoxic fraction increased with increase in tumor size in EMT6/KU tumors, while it reached a plateau at a certain size in SCCVII tumors.