Purification of cytokines was carried out while monitoring their in vivo anti-tumor activity and in vitro cytotoxic activities. As a result, purified new cytokines were obtained from culture supernatant of a histiocytic cell line and from rabbit serum. Briefly, a new cytokine (HSF, histiocyte-secreted-factor) was purified from the culture supernatant of the histiocytic cell line (TYH) which we established from the peripheral blood of a malignant-lymphoma patient. The purified samples exhibited suppressive effects on tumor growth but no necrotizing activity towards transplanted murine tumors. The substance displayed no cytotoxic activity against L cells (mouse fibroblast cells). The molecular weight of human HSF was about 42 kDa as estimated by SDS-PAGE. Amino-acid sequencing of the purified HSF from the culture supernatant was performed, but the N-terminal was blocked. Next, a new cytokine was purified from rabbit serum stimulated with Propionibacterium acnes and elicited with lipopolysaccharide. The rabbit HSF was isolated by the same procedures as those used for the human HSF purification steps. Amino-acid sequencing was carried out after enzyme digestion. Three parts of the amino-acid sequence of the rabbit HSF were determined as LPPGLLAPMRQLRS-, NLEXFTNGMEQHY-AQL-, and NPAENQAHELPNQLN-. A computer-based homology search demonstrated that these sequences were novel. The molecular weight of HSF as determined using anti-peptide antibodies revealed the following values: human HSF, 4 I and 46 kDa; rabbit HSF, 35,42 and 55 kDa.