Four human T cell lines, MT-2, TCL-Kan, TCL-As 2, and TCL-Haz, established from normal leukocytes by cocultivation with adult T-cell leukemia (ATL) virus (ATLV)-producing cells, produced constitutively phagocytosis inducing factor(s) (PIF) that induced phagocytosis in a human monocytic cell line, THP-1. These cell lines expressed ATLV-associated antigens (ATLA) as well as numerous virus particles, whereas the other twelve leukocyte cell lines tested, including T cell lines, B cell lines, and non-T and non-B cell lines, did not produce detectable amounts of the factor(s) in the culture supernatants. PIF was produced in the absence of serum and was not related to either ATLV-particles or viral structural proteins. Its activity was stable at 56C for 30min, but labile at 80C for 30min and at pH 2 for 20hr. MT-2 and TCL-Kan produced large amounts of the factor(s) in the culture supernatants but little interferon-γ (IFN-γ) or colony stimulating factor (CSF) activity was detected; furthermore, the activity was not neutralized by rabbit anti-IFN-γ sera. These observations suggest that some ATLV-transformed Tcell lines produce PIF that is different from IFN-γ and CSF.