To investigate the binding of interferon to human lymphoid cells, we purified human alpha interferon and radio-labelled it with iodine-125. Binding at 4 degrees C could be saturated and was inhibited by unlabelled interferon; it was specific for cells of human origin. Dissociation constants for the complex of interferon and receptor site were of the order 10(-9)-10(-11) M. All human cells tested showed such binding. Occupation of these high-affinity sites, at 37 degrees C, was compared with the inhibition of cellular growth due to interferon. The most sensitive cell line (Daudi) gave a complete biological response with only a fraction of its sites occupied. Evidence of two sites was found for a line (P3HRI) showing intermediate sensitivity. A relatively insensitive line (Raji) showed no response when all its high-affinity sites were occupied.