To evaluate the possible role of mycoplasma contamination of tissue cultures in virus induced chromosome breakage, human leukocyte cultures were inoculated with three mycoplasma strains; M . saliuarium, M. hominis type 2 and M. fernentans. All three strains caused mitotic inhibition when an inoculum of approximately lo6 CFU was used, the effect of M. fermentans being less severe than the one produced by the two other strains. Using a lower inoculum of lo3 CFU a n increase of chromosome breakage could be produced with M . saliuarium when the leukocytes were cultivated for five days. No chromosome changes were seen with M. hominis type 2 and M. fermentans.
The mitotic inhibition and chromosome breaks induced by M . saliuarium were discovered to be related to an arginine deficiency of the culture medium produced by the mycoplasma. This conclusion is derived from the fact that arginine addition was able to inhibit the mitotic inhibition and the increase of chromosome breakage, and secondly, that similar changes could be produced in leukocytes grown in arginine deficient medium without mycoplasma.
The growth of M . saliuarium was inhibited in leukocyte cultures treated with kanamycin, but the mycoplasma induced mitotic inhibition was still present, indicating that replication of mycoplasma organisms was not required for the production of arginine deficiency.