Synopsis A subline of the Yoshida ascites sarcoma which had acquired a very high resistance to nitrogen mustard (HN2) demonstrated the occurrence of cells with decreased HN2-resistance within the tumor. Through a repeated selection of lessresistant clones from that tumor, a successful isolation of entirely sensitive cells was achieved. The resistance of the parental tumor line persisted during six years of passage through animals in the absence of the chemical, although there was a marked

more »

... se in the degree of resistance on two occasions. Comparative chromosome analysis of the less resistant and non-resistant cells as well as the original sensitive cells showed a difference suggesting the possible association of chromosomal changes with the decrease or loss of resistance to the chemical.