Thyroid microsomes for use in assay systems for anti-thyroid-microsomal antibodies are normally prepared by ultracentrifugation. By means of gel filtration studies we show that preparations made in this way are invariably contaminated with thyroglobulin. A further purification by gel filtration is described which enables the preparation of thyroid microsomes of sufficient purity for use in a micro-ELISA method for the measurement of anti-thyroid-microsomal antibodies. Microsomes prepared in this way would also be suitable for haemagglutination and radioassays, which are also affected by thyroglobulin contamination.