This report concerns estimates of rheumatoid factor in human sera made by the sheep cell agglutination test. In an international collaborative assay of rheumatoid arthritis serum, estimates of potency of tested sera in relation to the International Reference Preparation were on average higher when made on the continent of Europe than when made in the United Kingdom and USA. In the present study, potencies were estimated in the National Institute for Medical Research using several sources of sheep cells in the United Kingdom and several from the continent of Europe. No large differences were found when other factors in the test were held constant.However, when the concentration of amboceptor used to sensitize the red cells was varied, considerable variation was noted in the estimated titre of rheumatoid factor. This confirmed earlier reports. Estimated potencies were also greatly affected by the concentration of amboceptor, the effect of amboceptor varying with the serum being tested; it was also found to vary with the source of sheep red cells that were sensitized.