Human repp86 becomes detectable in the nucleoplasm of cycling cells at the G(1)-S boundary, condenses at the centrosomes with the onset of mitosis, during which it progressively locates to the mitotic spindle and to the midbody, and vanishes at the completion of cytokinesis. The repp86 cDNA was cloned and sequenced. Full-length repp86 and its COOH-terminal domain cosediment with polymerized microtubules, linking repp86 to the family of microtubule-associated proteins. During prophase and metaphase, repp86 interacts on the mitotic spindle with the putative motor protein Hklp2. Thus, repp86 may function in targeting Hklp2 to the microtubule minus ends, its activity being regulated by phosphorylation of serine/threonine residues. Exogenous overexpression of repp86 provokes accumulation of cells in G(2)-M phase and subsequent polyploidization, suggesting that excess repp86 may interfere with correct nuclear division.