Three cell lines of mature T cell origin established from patients with cutaneous T cell lymphoma-leukemia (CTCL) have been found to be constitutive producers of TCGF (L-TCGF). Biologically active L-TCGF can also be eluted from the plasma membranes of these cells. We have compared the biologic and biochemical properties of L-TCGF and TCGF derived from normal lymphocytes (N-TCGF). L-TCGF and N-TCGF share similar biologic activity: both support long-term growth of T cells that have undergone prior lectin or antigen stimulation, and have no effect on unstimulated T cells. However, L-TCGF is a more acidic (pI 4.5 vs 6.5 to 8.0) molecule than N-TCGF and elutes from DEAE-Sepharose at higher salt concentration (0.2 M NaCl vs 0.07 to 0.1 M NaCl). In addition, these two factors display differing mobilities on gel filtration. Treatment of L-TCGF with neuraminidase or alkaline phosphatase does not alter its pI, indicating that enzymatically vulnerable sialic acid or phosphate groups are not involved in the variation. The nature and significance of this biochemical variant remain unknown.