Flow cytometry assessment of HIV receptor cell surface expression on U937 cells. Cell surface CR expression was increased after treatment with IFN-γ as determined by labeling with MIP-1α. Enhanced binding of MIP-1β was detected on cells pretreated with IFN-γ, as compared with untreated controls. CXCR4 expression remained unchanged, independent of pretreatment, as assessed with specific anti-CXCR4 monoclonal antibodies. CD4 expression on U937 cells was not altered by exposure to IFN-γ.