Radiopaque fluorocarbon (RFC), emulsions were prepared with small particle size and high concentration of the fluorocarbon. When RFC emulsions were injected i.v. in hamsters, rats and mice with 8 types of malignant tumors the tumors became radiopaque except in the mice with spontaneous teratoma of the ovaries. Tumors as small as 3-4 mm were defined radiographically using routine X-ray techniques. The tumors remained radiopaque for days to weeks after injection. Light microscopy and EM revealed characteristic fluorocarbon vacuoles primarily in the tumor macrophages. RFC emulsions may be useful in detection of malignant tumors.