Serial passage of the parotid gland tumor agent in mice. The activity of such tumor extracts for several strains of mice.

mice (6) when we repeated and confirmed some of the early experiments of Gross (8, 9). Our interest in these salivary gland tumors was stimulated greatly when we found that C58 and AK, as well as C8H mice, would develop such tumors following inoculation of newborn with cell-free extracts of leukemic AK mouse tissues (4). The parotid gland tumors in C58 or AK mice developed earlier than did those of the CSHanimals—usually afterlatentperiodsof 92—3 months. Coexistent leukemia, based on macro scopic and microscopic findings, was demonstrated only rarely at the time of tumor development. The parotid gland tumors induced in the several groups of mice (C3H/1He, CSH/fBi, AKR, C58) by AK leukemic extracts exhibited essentially similar histological patterns, varying only in the