Tetrameric motor proteins of the Kinesin-5 family are essential for eukaryotic cell division. The microscopic mechanism by which Eg5, the vertebrate Kinesin-5, drives bipolar mitotic spindle formation remains unknown. Here we show in optical trapping experiments that full-length Eg5 moves processively and stepwise along microtubule bundles. Interestingly, the force produced by individual Eg5 motors typically reached only ∼2 pN, one-third of the stall force of Kinesin-1. Eg5 typically detached from microtubules before stalling. This behavior may reflect a regulatory mechanism important for the role of Eg5 in the mitotic spindle. © 2007 EBSA.