Dose enhancement up to more than a factor of 100 was found in an environment of tissue-equivalent polymethylmethacrylate (PMMA) close to the surface of a thin metallic gold foil. The enhancement factors were determined for heavily filtered X rays (40 to 120 kV tube potential) under backscatter conditions, using thin-film radiation detectors with sub-micrometer resolution. The secondary electrons were found to range up to some 10 microm in tissue-equivalent material. Correspondingly, enhanced biological effects could be shown in vitro, using monolayers of C3H 10T1/2 mouse embryo fibroblasts exposed in intimate contact with the gold surface. The decay of the survival curves of cells irradiated on gold was significantly steeper than for those obtained from irradiation between PMMA disks with the same dose, also giving biological evidence for significantly enhanced doses at the gold interface. The shape of the inactivation curves resembled those for high-LET radiation, lacking a pronounced shoulder at the lower doses. Quantitatively, doses of e.g. 50 mGy (80 kV X rays) in homogeneous PMMA caused about 35% cell killing and 200 mGy about 80% when the cells were irradiated at the gold surface. From a comparison of these inactivation numbers with those found for irradiation between PMMA disks, biological dose enhancement factors for the cell system considered ranged up to about a factor of 50. In addition to cell inactivation, the in vitro irradiations of C3H 10T1/2 cells adjacent to the gold surface resulted in increased rates of oncogenic transformation. A dose of 100 mGy 80 kV X rays (measured in homogeneous PMMA) caused a frequency at an inserted gold surface comparable to that obtained with a dose of about 4.5 Gy of 60Co gamma rays in homogeneous PMMA.