Cryostat sections of normal human adult gastrointestinal mucosae were studied by double-label immunofluorescence with antibodies to CD3, CD4, CD8, CD5 and CD6, in parallel with antibodies fF1 and TCR61 against fl-chains and 6-chains of the T-cell receptor (TcR) types TcR2 (a/f) and TcRl (y/6), respectively. Virtually no TcRl + were found within the lamina propria. In the epithelial compartment, TcR1 + cells were infrequent: in the small bowel, -2% of T cells were TcR1 + . In the colonic epithelium, the percentage of T cells expressing y/6-chains was higher, with a mean value approximating 15-20%, although this apparently large percentage increase compared with small bowel reflects in part a much lower density ofcolonic IEL, as absolute numbers ofTCRbI + cells were comparable. Of the TcRl + population, about half were CD4-CD8-'double negatives' and the remainder were CD8+. TcRl + cells were also CD5-CD6-, irrespective of expression of CD8. No CD4+ cells expressing TcRl were observed: essentially all CD4+ cells were fF1+, with some variability of labelling intensity. Approximately 30-50% of the CD8+ subset expressed the PF1 antigen strongly. However, in the remaining TcRl -CD8+ cells, which were all of the CD5-CD6phenotype, expression of the flF1 antigen was only detectable when streptavidin and biotin conjugates were used for amplification of labelling. Thus, the CD8+ CD5subset, a prominent population of the epithelial compartment of the small bowel, was either TcR2dUII in the majority or TcRl + in a minority. Our data imply that y/6 TcRl cells may be actively excluded from intestinal lamina propria, and that any preferential localization that does occur is limited and is rather a feature of the colonic mucosa, rather than the small bowel.