The chemotactic responsiveness of monocytes from patients with serologically proven influenza infection has been quantified in vitro. Individuals with acute influenza had a significant (P less than 0-001) depression of monocyte chemotaxis. The depression ranged from 40% to 72% during acute infection but rose to normal by three weeks after recovery. When isolated mononuclear leucocytes from the recovered patients were incubated with the infecting strain of virus (Port Chalmers), a 49-54% inhibition of chemotaxis was obtained. These findings support the hypothesis that the altered immune responsiveness and increased predisposition to superinfections found frequently in patients with influenza can be due to the ability of the virus to depress monocyte function.