Abstract. Centriole stability and distribution during the mammalian cell cycle was studied by microinjecting biotinylated tubulin into early G~ cells and analyzing the pattern of incorporation into centrioles. Cells were extracted and cold treated to depolymerize labile microtubules, allowing the fluorescent microscopic visualization of the stable centrioles. The ability to detect single centrioles was confirmed by use of correlative electron microscopy. Indirect hapten and immunofluorescent labeling of biotinylated and total tubulin permitted us to distinguish newly formed from preexisting centrioles. Daughter centrioles incorporated biotinylated tubulin, and at mitosis each cell received a centrosome containing one new and one old centriole. We conclude that in each cell cycle tubulin incorporation into centrioles is conservative, and centriole distribution is semiconservative.