A noninvasive in vivo bioluminescence of a spontaneous tumor arising in a mouse in which both alleles of the retinoblastoma gene have been inactivated by Cre-mediated recombination in the pituitary gland. Pituitary cells and, consequently, the pituitary tumor cells express a luciferase transgene. The emitted photons are a measure for the amount of luciferase present in living cells and, therefore, directly correlate with the viable tumor mass. The emitted photons are captured by a sensitive-cooled CCD camera. This approach permits accurate longitudinal monitoring of tumor growth, stasis, and regression in response to experimental prevention and therapeutic intervention protocols. For details, see the article by Vooijs et al. on page 1862 of this issue.