A temperature-sensitive (ts) mutant, tsBN2, which was derived from BHK21 and is defective in the regulatory mechanism for chromosome condensation, was transformed to the temperature-resistant (ts+) phenotype by means of DNA-mediated gene transfer with hamster and mouse DNA. Treatment of mouse DNA with the restriction enzymes EcoRI, HindIII, PstI and SalI, but not with XhoI, almost completely abolished the transforming activity. A fluctuation test, originally devised by Luria and Delbrück, was used to estimate the reversion and transformation frequencies of tsBN2 cultures.