Summary Three sublines of HeLa cells were infected with Mycoplasma orale type 1, M. hominis and M. hyorhinis and compared with mycoplasma-free cells for their ability to support the growth of adenovirus types 2 and 31, herpes simplex virus, Sabin's poliovirus type 1, parainfluenza virus type 2, respiratory syncytial virus and vaccinia virus. Titres of herpesvirus and adenovirus tended to be reduced in the cells infected with mycoplasmas, but titres of the other viruses were generally raised by such infection. The largest differences in titre in either direction were approximately ten-fold and were obtained with adenovirus type 2 (reduction) and vaccinia virus (increase). Much smaller effects were noted with the other viruses. With herpesviruses and adenoviruses, which have a known requirement for arginine, the antagonistic effect of the two arginine-metabolising mycoplasmas, M. orale type 1 and M. hominis, was more than that of M. hyorhinis, which does not metabolise arginine. With the other viruses, the higher titres obtained in the mycoplasma-infected cells were independent of the species of mycoplasma. The cell lines infected with M. hyorhinis were difficult to maintain for long periods on account of the cytopathic effects of this species, and were, therefore, less suitable for titration of the more slowly growing herpes- and adeno-viruses. It is concluded that mycoplasma contamination of a HeLa cell line may interfere with the titration of some viruses, but the effects are relatively small and probably do not constitute a serious hazard in routine diagnostic virology.