The National Institute for Medical Research, London, England, was requested by the WHO Expert Committee on Biological Standardization to arrange a collaborative study of the serum pool they had obtained, to determine its suitability to serve as an international reference preparation of rheumatoid arthritis serum. A batch of this serum was assayed by 11 laboratories in 7 countries against 30 test preparations. On the basis of the results obtained, the serum has been established as the International Reference Preparation of Rheumatoid Arthritis Serum and the International Unit of Rheumatoid Arthritis Serum has been defined as the activity contained in 0.171 mg of the international reference preparation.A description is also given of the British reference preparation of rabbit antibody to sheep red blood cells (amboceptor) and this material was also tested in the collaborative study.