Cell banks were created by controlled rate freezing of pooled cell suspensions. Each line was characterized for freedom from adventitious agents, identity and biological behavior in immunosuppressed animals. Pharmacopeal culture methods were employed to search for bacterial, fungal and mycoplasma contamination. Conditioned medium was inoculated into a battery of 6 cell lines to search for acute viral contamination. Recombinant cell lines were also examined for retroviruses by electron microscopy, reverse transcriptase assay, XC plaque assay, tritiated uridine labelling, chemical induction techniques, and co-cultivation with three indicator cell lines. Further characterization was obtained from inoculation of 4 animal species in vivo, mouse antibody production test, and inoculation of immunosuppressed rodents. Finally, cytogenetic evaluation including banding methods and isoenzyme analysis were employed to verify cell bank identity. These methods have generated useful data concerning the identity, safety, and biological purity of recombinant DNA mammalian cell lines.