During routine lower gastrointestinal endoscopy of children for suspected chronic inflammatory bowel disease, it is possible to visualize lymphoid follicles (Peyer's patches) in the last few centimeters of the terminal ileum. Biopsy specimens have been taken from these Peyer's patches and the lymphoid cells have been isolated by collagenase digestion. The mean cell yield of Peyer's patch cells was 1.7 x 10(8) lymphocytes/g tissue. Isolated Peyer's patch cells were 26%-54% CD3+ (pan T cells), 14%-34% CD4+ (helper/inducer T cells), and 9%-17% CD8+ (suppressor/cytotoxic T cells). Twenty six percent to 48% of the Peyer's patch cells were B cells; less than 1% contained cytoplasmic immunoglobulin A. When stimulated in vitro with phytohemagglutinin or pokeweed mitogen, cells from Peyer's patches proliferated. The successful isolation of functional cells from Peyer's patches will now allow studies to be done at the afferent limb of mucosal immunoregulation in humans.