L-3,5,3′-triiodothyronine (T3) has been measured by a radioimnunoassay the special features of which permit determinations in unextracted serum. These include the use of tetrachlorthyronine which appears to inhibit the binding of To to thyroxine binding globulin but does not affect the binding of T3 to antibody and utilization of a T3-free serum prepared in vitro for the construction of a serum standard curve. Mean T3 levels were 60 ng % in hypothyroidism, 139 ng % in normal subjects and 587 ng % in hyperthyroidism.