A parotid gland tumor agent derived from tissues of an AKR mouse which had developed spontaneous leukemia was passed in series through fifteen inbred C3H/fBi or AKR mice. Such passage was maintained by inoculation of newborn mice with cell-free extracts, or implantation of tumor tissue into young adult mice 4–6 weeks of age. One tissue culture passage was included. The latent period was not progressively reduced. Histological studies indicated a close similarity of pattern in the parotid gland and subcutaneous tumors which resulted from implantation of tissue. The tumor-inducing activity of extracts of parotid gland tumors (extract induced) and subcutaneous tumors (tissue implanted) was measured by inoculation of newborn mice of strains AKR, C3H, and Swiss. Parotid gland tumors developed in AKR mice which received AKR or C3H tumor materials, while C3H mice exhibited only occasional sarcomas. Swiss mice developed mammary tumors in 6–7 months, extracts of which produced mammary tumors as early as 3 months after inoculation of newborn Swiss mice.