Recent studies have shown that normal human T cells contain a high- molecular-weight (mol wt) protein exhibiting B cell growth factor (BCGF) activity. Other studies have shown that virally transformed human B cells also secrete a high-mol-wt BCGF-like molecule in vitro. We have studied neoplastic B cells from patients with untreated hairy cell leukemia (HCL) to ascertain whether such cytoplasmic BCGF activity is present in the tumor cells. Studies on HCL cells from four patients indicated that BCGF-like activity was in fact present in the cytosolic extracts when tested on autochthonous HCL cells as well as on normal BCGF-dependent human B cell lines. Chromatographic analysis indicated that the BCGF activity from HCL cells was similar in mol wt as well as function to the normal T cell-derived cytosolic BCGF activity. These studies suggest that HCL cells contain and, in some cases, secrete a high-mol-wt growth factor that can be autostimulatory and appears to resemble a similar growth factor molecule found in normal human T cells.