A cDNA encoding for human monocyte chemotactic and activating factor (MCAF) was ligated into the retroviral vector pLXSN. These pMCAF-LXSN and antisense p-antiMCAF-LXSN vectors were transfected into HBT20 and HBT28 human brain tumor cells. HBT28 cells constitutively express high amounts of MCAF, whereas HBT20 cells express much less MCAF. HBT20 cells transfected with pMCAF-LXSN (HBT20-MCAF) showed significantly higher MCAF mRNA expression and MCAF protein production than the HBT20-parent or HBT20 cells transfected with control vector (HBT20-LXSN). In contrast, supernatant from HBT28 cells transfected with p-antiMCAF-LXSN (HBT28-antiMCAF) contained less MCAF than HBT28-parent, HBT28-LXSN, and HBT28-MCAF cells. Activated human monocytes killed HBT20-MCAF cells more efficiently compared with HBT20-parent, HBT20-LXSN, and HBT20-antiMCAF cells (P< 0.02), whereas HBT28-antiMCAF cells were killed more efficiently by activated monocytes compared with HBT28-parent, HBT28-LXSN, and HBT28-MCAF cells (P< 0.05). Cultured supernatants from activated monocytes plus HBT20-MCAF cells or from activated monocytes plus HBT28-antiMCAF cells inhibited the growth of HBT20 and HBT28 cells, respectively. Altered MCAF expression can therefore enhance the ability of activated monocytes to kill brain tumor cells. This increased cytotoxicity is partially dependent upon the basal state of MCAF in the individual tumor cells.