The Ig-containing cell population of human intestinal mucosa (jejunum and ileum) has been quantitatively studied in 15 normal healthy adult men, by means of the peroxidase-antiperoxidase (PAP) method. The resolution obtained using the PAP method improves that obtained by fluorescence microscopy methods. The results obtained revealed that the number of Ig-containing cells progressively decreases from the basal portion of mucosa to the tip of the microvilli. Most Ig-containing cells were plasma cells; however, stained precursor cells (immunoblasts) were also observed in the human intestinal mucosa. The epithelial lining of the intestinal mucosa was also stained by anti IgA, anti IgG, anti IgM and anti IgE antibodies. Plasma cells were not seen to cross the epithelium. The absolute values, expressed as the number of Ig-containing cells per "mucosal tissue unit" (a 6-micron-thick and 500 micron wide block of tissue, including the mucosa at full height from the muscularis mucosa), and the relative percentages of the different Ig-containing cell population were the following: IgA: 86.71 +/- 15.58 cells (48.52%); IgG: 52.30 +/- 19.01 cells (29.6%); IgM: 21.44 +/- 8.23 cells (12.0%); IgE: 14.70 +/- 6.60 cells (8.22%), and IgD: 3.54 +/- 1.05 cells (1.9%). The number of lambda chains (76.29 +/- 24.38; 56.07%) were slightly more abundant than that of chi chains (59.77 +/- 11.93; 43.92%). The differences between the mean values of different Ig-classes were significant, and the differences between lambda and chi chains were also significant.