To determine whether GM-CSF also enhances cytokine production in human monocytes, we pretreated undifferentiated monocytes derived from the human monocytic cell line, THP-1, with recombinant GM-CSF prior to infection with Legionella or stimulation with the Toll-like receptor 2 (TLR2) agonist Pam3CSK4. Here, CSF2 is linked to infection.