While the literature on rapid sedimentation rates is exceedingly large, clinical papers dealing with slow sedimentation rates are conspicuous by their absence. We found fast rates in the majority of 610 unselected patients in whom the speed of sedimentation was, in many cases repeatedly, determined. Almost 20 per cent of these patients had normal or moderately slow rates, and 115 patients (19 per cent) had much slower rates than healthy subjects. As time went on, it became evident that a large proportion of these slow rates (46 per cent) was obtained in patients with various generally recognized or probable manifestations of allergy. Patients with vague symptoms predominated among the other half. Although the determination of the sedimentation rate was only part of a study of various physicochemical changes of the blood, it seemed worth while to analyze this observation separately, especially because the clinical significance of the increased suspension stability