SERUM protein and immunoglobulin concentrations were measured in children with acute rheumatic fever (RF) and rheumatoid arthritis (RA). In the course of our experiment we were intrigued to find two girls in the RA group with a complete absence of serum IgA. The case summaries of these two patients and the results obtained from the two study groups as a whole are the subjects of this report. Patient Selection and Classification Rheumatoid Arthritis (23 patients).—Group 1 included children attending the pediatric rheumatoid clinic at the North Carolina Baptist Hospital (NCBH). They were classified on the basis of the criteria of Grokoest et al.1 Acute Rheumatic Fever (23 patients).—Group 2 included children who were either inpatients on the pediatric service at the NCBH at the time of diagnosis, or were being followed in the rheumatic fever clinic. All had definite diagnoses of acute RF at the time of