A rapid radioimmunoassay for serum myoglobin concentration has been developed, capitalizing on a technique for rapid separation of antibody-bound antigen from free antigen using Sepharose-bound sheep anti-rabbit gamma-globulin. Total time required for the assay is 2.75 h, making it suitable for clinical approaches employing myoglobin for rapid identification of patients in the active phase of myocardial injury.