The mean concentration of IgA in the mesenteric lymph of guinea pigs and rats was 4.5 and 13.4‐fold higher, respectively, than that in their serum. For IgG, IgM, albumin, transferrin and α‐macroglobulin, the mesenteric lymph‐versus‐serum concentration ratios were lower than unity and inversely related to their molecular size, indicating that their presence in mesenteric lymph was essentially due to filtration through the walls of the mesenteric blood capillaries. In contrast, it was calculated that 90–97 % of IgA in mesenteric lymph was derived from local synthesis by the intestinal immunocytes. Considering mesenteric lymph flow and plasma volume, it was concluded that the intestinal mucosa is a major source of serum IgA in guinea pigs and rats.