Interstitial cells of Cajal (ICC) generate the pacemaker component of the gut and play important roles in the control of gut motility. The tyrosine kinase receptor Kit is an established marker for ICC. Recently, it has been reported that immunoreactivity for the sialomucin CD34 may be present on ICC in human intestine. Gastrointestinal stromal tumors express both Kit and CD34, suggesting that these tumors may derive from ICC. We characterized the distribution of CD34 immunoreactivity at the cellular level in the normal human gut, using double immunofluorescence immunohistochemistry and confocal microscopy. CD34 immunoreactivity identified previously unrecognized cells closely adjacent to, but distinct from, the Kit immunoreactive ICC. These CD34 immunoreactive cells expressed the fibroblast marker prolyl 4-hydroxylase-whereas ICC did not-and were also distinct from smooth muscle cells, glial cells, and macrophages. In the human gut, CD34 immunoreactivity is not expressed by ICC but by a population of fibroblasts, likely corresponding to the "fibroblast-like cells" described in previous ultrastructural studies. Our findings also challenge the hypothesis that stromal tumors originate from ICC. (Lab Invest 1999, 79:59-65).