The aim of this work has been to isolate and characterize new diploid cell strains, suitable for large-scale production of human fibroblast interferon. Twenty cell strains were isolated from individual neonatal foreskins obtained with the informed consent of the donors' parents. The techniques employed for the isolation of the cell strains were aimed at obtaining the highest possible yield of normal diploid cells, free of contaminating microorganism and viruses. The bulk of the cell yield has been frozen at a low population doubling level. Each of the isolated cultures was tested for interferon producing characteristics with poly(I)-poly(C) under a number of different conditions including "superinduction" with metabolic inhibitors. Most of the newly established cell strains produced lower interferon yields than the reference FS-4 cell strain. However, some new cell strains produced similar interferon yields as the FS-4 cells on superinduction. Five cell strains, designated FS-30, FS-35, FS-44, FS-48 and FS-49, identified as the highest interferon producers among the new cells, were selected for further testing. Of these, three cell strains (FS-35, FS-48 AND FS-49) produced similar interferon yields as FS-4 cells after superinduction. Cell strains FS-48 and FS-49 were found to have stable interferon producing characteristics over a wide span of population doubling levels. The interferon produced in these new cell strains had the antigenic and biological characteristics of human fibroblast interferon.