We determined the susceptibility of CD4+ and CD8+ subsets of T lymphocytes to infection, and whether monocytes-macrophages were required for the uptake of influenza virus by lymphocytes, or merely for the activation of the lymphocytes to a state (similar to that of mitogen-stimulated cells) that supported the synthesis of viral proteins after independent uptake of the virus by those cells. This evidence concerns the gene CD4 and infection.