One hundred patients in whom an Erythrocyte Sedimentation Rate (ESR) in excess of 100 mm. in the first hour was found on 2 consecutive occasions were investigated. Serum protein electrophoresis was performed on 96 of these patients and bone marrow examination on 55 patients. Acute infection was the commonest diagnosis though the majority of patients had 2 or more separate conditions each contributing to the elevation of the ESR. Quantitive serum protein electrophoresis was abnormal in all but one patient and was of limited diagnostic value. A definite band in the globulin region was detected in 11 patients, 7 of whom were found to have myelomatosis. Bone marrow examination was useful only in patients with a discrete band in the globulin fraction or with a specific haematological abnormality. It is suggested, therefore, that bone marrow examination be confined to patients with such abnormalities irrespective of an elevation of their ESR.