Objectives: The possibility of leakage of CD34+ bone marrow stem cells from the peripheral blood into the coelomic cavity and the capability of coelomic fluid factors to induce their non-hematogenous differentiation were examined by immunocytochemistry (ICC). Study Design: Body cavity fluid smears from 12 and 18 patients with and without cancer, respectively, were processed for double immunoperoxidase or double fluorescent ICC methods using antibodies against CD34, CD14, CD16, CD68, AE1/AE3, epidermal growth factor receptor (EGFR), D2-40, and CA125. Results: Heavily irritated exudative fluid from 6 patients with or without cancer contained a few small round cells positive for CD34. Some of them co-expressed myeloid or monocytic markers such as CD14, CD68 or CD16. Some of the CD34+ cells also co-expressed AE1/AE3 or EGFR. In addition, D2-40 and CA125 were also demonstrated though the expression of the latter was quite sporadic. Conclusion: These findings support the concept that CD34+ stem cells can be released into irritated body cavity fluid and the possibility of subsequent differentiation to a non-hematogenous lineage under the influence of local humoral factors, in agreement with our previous in vitro experiments. The possibility of such a phenomenon should be kept in mind when body cavity fluid specimens are analyzed by ICC for diagnostic purposes.