A quantitative analysis of the binding kinetics of intact Moloney murine leukemia retrovirus (MoMuLV) particles with NIH 3T3 cells was performed with an immunofluorescence flow cytometry assay. The virus-cell binding equilibrium dissociation constant (K D ), expressed in terms of virus particle concentration, was measured to be 8.5 (؎6.4) ؋ 10 ؊12 M at 4؇C and was three-to sixfold lower at temperatures above 15؇C. The K D of virus binding is about 1,000-fold lower than the K D of purified

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... envelope. The association rate constant was determined to be 2.5 (؎0.9) ؋ 10 9 M ؊1 min ؊1 at 4؇C and was 5-to 10-fold higher at temperatures above 15؇C. The apparent dissociation rate constant at 4؇C was 1.1 (؎0.4) ؋ 10 ؊3 min ؊1 and was doubled for every 10؇C increase in temperature over the range tested (15 to 37؇C).