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Sera from a random sample of the population of Rotterdam were tested by a latex fixation test (LFT) in Leiden, a sensitized sheep cell agglutination test (SCAT) in Manchester and a human erythrocyte test (HEAT) in Groningen with an overall agreement of 93.3 per cent. I n both rheumatoid and nonrheumatoid subjects sera were encountered which were positive in the cell tests but negative in the LFT and vice versa. PIDEMIOLOGIC studies of rheumatoid arthritis have been carried out E by various workers in the USA. and Europe. In order to compare the findings, some assessment of observer differences is required, and this has been done with regard to the interpretation of radiologic data.1-3 For similar reasons it seemed important to compare the serologic results obtained bv different laboratories engaged in survey work. Accordingly sera from a randnm sample of the population of Rotterdam were tested by an F.11 latex-fixation procedure in Leiden, and by two tests utilizing immune rabbit serum, namely, a sheep cell agglutination test in Manchester and a human erythrocyte test in Groningen. MATERIAL AND METHODSSera from 138 men and 135 women were collected by Dr. R. de Graff during a survey of the southern part of Rotterdam (Netherlands), This population sample included only the age group 55-64 years. In assessing clinical and radiologic data, the criteria of the American Rheumatism Association4 were used for the diagnosis of rheumatoid arthritis. Latex Fixation Test (LFT)Non-inactivated serum was titrated with latex particles (Dow Chemical Co., Inc.), "coated" with fraction I1 -,+globulin (derived from human plasma pools) as supplied by the Netherlands Red Cross. Glycine buffer (0.1 M glycine in 0.6 per cent NaC1) was used as a diluent. The tests were incubated at room temperature for 20 hours after incubation From the

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