Two continuous lines, BL-MaTU/A1 and BL-MaTU/s6, were established from C57Bl/10 mammary adenocarcinomas induced by DMBA-prolactin-estradiol treatment. Under in vivo stimulation with dexamethasone, insulin, prolactin, and prostaglandin A1, the cells produce detectable amounts of B-type particles with biochemical properties similar to the GR-MuMTV. Analysis of restriction endonuclease-generated fragments of cellular DNA revealed identical patterns in the integration sites and internal recognition sites of MuMTV proviral equivalents in the tumor cells and normal organs of C57Bl/10 strain mice. The restriction DNA fragments of C57Bl/10-associated MuMTV proviral DNA are closely related to those of the Balb/c-associated MuMTV. These results indicate the endogenous origin of MuMTV produced in the hormonally stimulated cultures of DMBA-prolactin-estradiol-induced C57Bl/10 mammary tumors.