We measured the amounts of interferon (IFN) induced by four wild-type strains and two attenuated vaccine strains of poliovirus, testing each virus in 12-58 separate batches of human peripheral blood leukocytes. There were big and consistent differences in the mean amounts of IFN formed by leukocytes from different donors in response to all these polioviruses. Almost invariably, the wild-type polioviruses gave rise to more IFN than the corresponding attenuated strains, but overall, the amounts induced by one strain of poliovirus were proportional to those induced by another. There were indications of similar correlations with six different influenza viruses and with three strains of herpes simplex virus. In contrast, there was no correlation between the IFN yields induced with a poliovirus, three other enteroviruses, and various other viruses. These results suggest that the leukocytes from a given individual have a capacity to form IFN in vitro that is determined both by the genetic make-up of that individual and the particular virus concerned. If the same phenomenon applies to IFN production by cells in vivo, it may have a role in pathogenesis of viral infections.