The use of human serum albumin labelled with iodine-131 (RIHSA) as the radioactive indicator in the measurement of cardiac output by the external counting technique is not ideal since the emission characteristics, physical half-life, and physiological fate restrict the amount that can be administered. A more suitable material, especially because of the short physical half-life of the isotope involved, is albumin labelled with technetium-99m. This communication describes a simple three-stage process for the preparation of such a complex which was then tested in subjects with no impairment of cardiac performance for its suitability in assays of cardiac output by the external counting method. Values were within the same range as those obtained with RIHSA in comparable subjects. Blood volumes estimated with 99mTc-albumin were within the limits of physiological variation of values derived with RIHSA in the same subjects. Serial blood samplings and urine collection during a period of 24 hours after administration showed that the preparation was lost continuously from the circulation at a more rapid rate than RIHSA. The greatest loss was in the first few hours when most of the urinary excretion of the isotope occurred. Comparison of the present data with published results using an alternative preparation suggests that our product may be more stable.