A highly sensitive radioimmunoassay (RIA) for T3 has been developed in which ethanol extracts of serum are incubated with antiserum and tracer in order to minimize serum protein interference. Recovery by this method was complete and reproducible. Assay sensitivity ranged from 6–14 ng/100 ml. Mean serum T3 concentrations (ng/100 ml) in normal and abnormal thyroid states were as follows: 130 (normal males); 127 (normal females); 172 (normal females on oestrogens); 450 (hyperthyroidism); 42 (severe primary hypothyroidism; 127 (borderline primary hypothyroidism); 38 (secondary hypothyroidism). This simple procedure is suitable for routine clinical use and has the potential for simultaneous T3 and T4 measurements.