The spindle checkpoint prevents errors in mitosis. Cells respond to the presence of kinetochores that are improperly attached to the mitotic spindle by delaying anaphase onset. Evidence suggests that phosphorylations recognized by the 3F3/2 anti-phosphoepitope antibody may be involved in the kinetochore signaling of the spindle checkpoint. Mitotic cells lysed in detergent in the absence of phosphatase inhibitors rapidly lose expression of the 3F3/2 phosphoepitope. However, when ATP is added to lysed and rinsed mitotic cytoskeletons, kinetochores become rephosphorylated by an endogenous, bound kinase. Kinetochore rephosphorylation in vitro produced the same differential phosphorylation seen in appropriately fixed living cells. In chromosomes not yet aligned at the metaphase plate, kinetochores undergo rapid rephosphorylation, while those of fully congressed chromosomes are under-phosphorylated. However, latent 3F3/2 kinase activity is retained at kinetochores of cells at all stages of mitosis including anaphase. This latent activity is revealed when rephosphorylation reactions are carried out for extended times. The endogenous, kinetochore-bound kinase can be chemically inactivated. Remarkably, a soluble kinase activity extracted from mitotic cells also caused differential rephosphorylation of kinetochores whose endogenous kinase had been chemically inactivated. We suggest that, in vivo, microtubule attachment alters the kinetochore 3F3/2 phosphoprotein, causing it to resist phosphorylation. This kinetochore modification is retained after cell lysis, producing a "memory" of the in vivo phosphorylation state.