The aim of this study was twofold. First, to establish quantitatively the distribution of the immunoglobulin-containing (plasma) cells, T and B lymphocytes in the lamina propria of the rectal mucosa of normal neonates and neonates with Hirschsprung's disease (HD). Second, to review the neonates with HD to determine any differences in these cell populations between those who subsequently developed Hirschsprung's enterocolitis (HEC) and those who did not. Two conclusions can be drawn from the results of our study of rectal mucosal immune defenses. First, neonates with HD have no deficiencies in these defenses when compared with normal neonates. Second, neonates with HD who subsequently develop HEC have no premorbid deficiency in these defenses. It was noted that the pan-T cell count in the infants who went on to develop HEC appeared to be increased, although this did not reach statistical significance. The use of fresh or frozen material would permit a more detailed analysis of the separate T cell subsets.