Summary: In 46 intestinal specimens from infants 2 hr to 6 months old, the numbers of immunoglobulin-containing cells were counted by the direct immunofluorescent or direct immunoperoxidase technique. A restudy was made of biopsy specimens taken for diagnostic purposes, 34 from the rectum and 3 from the transverse colon of 30 infants at the Children's Hospital, Helsinki in 1971–1977, and in addition, of autopsy material recently collected from the intestines of 9 infants. Up to the age of 12 days, no immunoglobulin-containing cells were seen. A small number (1 to 2) of IgM-containing cells was seen in the lymph nodes of the 2 earlier specimens taken at the ages of 6 and 12 days. A rectal specimen from a 12-day-old infant showed 24 IgA- and 60 IgM-containing cells/mm2. In rectal specimens of infants less than 1 month old, the mean number of IgM-containing cells (26/mm2) was higher than that of IgA-containing cells (14/mm2), but older infants had a significantly higher mean number of IgA-containing cells (P < 0.01). The mean number of IgM-containing cells was the same in children 1 to 3 months (53 cells/mm2) and 3 to 6 months of age (59 cells/mm2), whereas the mean number of IgA-containing cells increased with age up to 6 months (112 and 163 cells/mm2). The youngest infant who had IgG-containing cells was 13 days old, although positive staining of intercellular spaces in the lamina propria and of the capillary endothelium by anti-IgG serum was observed in all specimens. The mean number of IgG-containing cells was low (5 cells/mm2) in all age groups. Sparse IgE-contain-ing cells (less than 12/mm2) were seen in 4 of the 46 specimens. In 5 patients, 2 or 3 consecutive specimens were available for the study; in these, the increase in the numbers of IgA- and IgM-containing cells was similar to the mean increase in cell numbers for the series as a whole. Speculation: There is a deficiency of immunoglobulin-containing cells in the gut of newborn infants up to the age of 12 days, and this is probably the reason for the high permeability of the neonatal intestine to foreign proteins.