In a cross-sectional study of over 3000 consecutive serum specimens the levels of rheumatoid factor (RF) measured by rate nephelometry (Beckman ICS II) were compared with values obtained by the more traditional methods of sheep cell agglutination (Rose-Waaler) and latex agglutination. Similar values for sensitivity and specificity were found for all three methods for rheumatoid arthritis, with nephelometry giving slightly higher levels of sensitivity for other rheumatic disorders. A significant correlation (r = 0.46, p less than 0.01) was found between the nephelometric and Rose-Waaler method for 147 consecutive seropositive specimens. Of interest, however, several disparate results were observed, and explanations for these were sought. Longitudinal studies of RF were performed in 49 seropositive patients over a two-year period. The nephelometric method was considered superior compared with the other techniques because of its ability to detect changes in absolute levels at earlier stages and its low interassay coefficient of variance (11%). We conclude that the nephelometric technique appears suitable for routine diagnostic use, offers several advantages compared with more traditional methods, and is no more expensive per test specimen than the Rose-Waaler technique.