A survey of human diploid, aneusomic, transformed and tumor fibroblast or fibroblastoid cell lines for their capacity to produce interferon after polyriboinosinic acid:polycytidylic acid induction showed considerable variation in responsiveness. There was no apparent correlation between karyotype or phenotype and interferon production. Pretreating or "priming" the cells with human interferon generally led to increased yields of interferon after polyriboinosinic acid:polycytidylic acid induction in all cell lines tested. All the cells also showed the "super-induction" phenomenon, although to varying degrees. The combination of priming and superinduction conditions led to the production of very high yields of interferon in some cell lines, but in other lines, yields less than either the primed or superinduced amounts were found. A more limited survey of human cell lines for their capacity to produce interferon after Newcastle disease virus induction also showed that yields varied from line to line. However, there was little evidence to suggest that ability to produce interferon after Newcastle disease virus induction correlated with that after polyriboinosinic acid:polycytidylic acid induction.