A radioassay for determination of thyroglobulin antibodies in human serum using [125I]thyroglobulin co-precipitated with antihuman IgG is described. Serial dilutions of the antibody containing sera gave nearly rectilinear and parallel logit-log curves in conditions of moderate antigen excess. A secondary standard serum calibrated against the Medical Research Council Research standard A 65/93, which by definition c;ntains 1 Mega unit/1 (MU/1) was used for standardization. The mean imprecision in the concentration range 0.74-241 MU/1 was CV = 3% (within assay) and CV = 8% (total). The detection limit was 0.002 MU/1. The assay was compared to an antigen binding capacity method with an imprecision of 15% (total) and a detection limit of 0.1 MU/1. The coefficient of correlation between the two methods was: R = 0.997 (our method = 0.019 x antigen binding capacity -0.33). Based on this 1 Mega unit was found equivalent to 53 nmol thyroglobulin.