A method is described for the simultaneous measurement of triiodothyronine (T3) and thyroxine T4) in 0.04 ml of unextracted serum. Antibodies were prepared by immunization of rabbits with T3 and T4 conjugated with human serum albumin. Bound and free labeled hormones were separated by the double-antibody technique, and 8-anilino-1-naphthalene sulfonic acid was used to inhibit binding of the two hormones to thyroxine-binding globulin. The validity of the assay using 125I-T3 and 131I-T4 is shown by the excellent recovery of T3 and T4 added to serum and by the finding that curves obtained by assaying various dilutions of a hyperthyroid serum run parallel to the standard curves. In all clinical serum T3 and T4 values obtained using the double-tracer radioimmunoassay method were in excellent agreement with those obtained by single-tracer RIA techniques. The combined T3-T4 method appears to be accurate, sensitive, and specific, thus making the assay highly desirable as a technical time-saver.