Immunoglobulin A has long been referred to as the antibody of mucosal secretions. However, in the last few years there has been increased interest in serum IgA, in factors that regulate the various forms of IgA produced, and in the relationship between serum and mucosal IgA. Recent data indicate that in humans, the mucosal surfaces are neither the source nor the destination of serum IgA. By estimating the amounts of IgA produced at various sites throughout the body, we have shown that in humans more IgA is synthesized and secreted each day than IgG and IgM combined; about one third of this IgA is secreted directly into the vascular compartment and never reaches the mucosal surfaces. We also consider a possible role for serum IgA as the "discreet housekeeper," the relationship between cells that produce IgA in the serum and IgA in secretions, and factors that influence the various forms of IgA produced.