[en] The production of ¹⁹¹Os for ¹⁹¹Os-/sup 191m/Ir generator by irradiation of natural osmium has been assessed because of limited supplies of enriched ¹⁹⁰Os. The distribution of radionuclide products has been determined after a 14-day irradiation of natural osmium (26.4% ¹⁹⁰Os) in the High Flux Isotope Reactor (HFIR) to evaluate the potential usefulness of this method of ¹⁹¹Os production in comparison to irradiation of 97.8% enriched ¹⁹¹Os. Studies clearly indicate that neutron irradiation of natural osmium is impractical for production of ¹⁹¹Os for clinical use and that irradiation of enriched ¹⁹⁰Os is the preferred production mode. A new tellurium fatty acid has been prepared and evaluated in rats in which radioiodine (¹³¹I) has been chemically stabilized on the molecule as a vinyl iodide [I-CH=CH₂-(CH₂)₃-Te-(CH₂)₁₁-COOH]. The absolute heart uptake (0.35 to 0.77% injected dose/gram) was only marginal, however, suggesting that this particular vinyl iodide tellurium fatty acid does not show high heart specificity. Several radiolabeled agents were prepared and distributed to Medical Cooperative investigators for further preclinical study and clinical evaluation