Expression of mammary tumor virus (MTV) antigen was measured in a wide variety of organs and tissues of a series of high (GR, SHN, SHNf, DD, SLN, SLNf) and low (DDf, DDD, DDDf, KF, KFf, ddY, C57BL, BALB/c) mammary cancer strain mice. Tests were carried out by microimmunodiffusion (micro-ID) and immunoperoxidase tests on formalin-fixed tissues and radioimmunoassays in extracts for 2 viral proteins, MTVp27 from the viral core and MTVgp52 from the viral envelope. Organs with exocrine function, i.e. the mammary gland, salivary gland, coagulating gland and prostate, were mostly positive. The secretory epithelial cells of these organs showed viral antigen expression. Less positivity was encountered in brain, pancreas, stomach, urinary bladder, epididymis, uterus, thymus, spleen, lymph nodes and kidney, plasma and blood cell pools. Unexpectedly the uterus extracts showed MTV antigen expression, occasionally even by immunodiffusion, especially in mice of various DD stocks, but also in the GR strain. Another striking observation was the detection of MTV antigen expression in salivary glands in C57BL strain mice; most other organs of this strain (including the mammary glands) were negative. The implications of results of this extensive survey for MTV antigen expression are discussed for tumorigenesis by MTV of mammary gland and perhaps other tissues in the mouse.