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In the present work we have studied the subunit composition of kinesin, the microtubule‐activated, mechanochemical ATPase, isolated from bovine brain. Polypeptides with mol. wts of 120 and 62 kd are the major components of the kinesin preparation. These polypeptides could not be separated by electrophoresis under nondenaturing conditions or by FPLC on a MonoQ column, and are therefore assumed to form a tight complex. As shown by immunoblotting with polyclonal and monoclonal antibodies to the 120‐kd polypeptide and by one‐dimensional peptide mapping, the 62‐kd polypeptide does not appear to be a proteolytic product of the 120‐kd component. Densitometric scanning of polyacrylamide‐SDS gels shows that these polypeptides are present in a complex in a 1:1 molar ratio. The mol. wt of native kinesin was studied by sedimentation equilibrium and was found to be 386 +/‐ 14 kd. A comparison of the mol. wts of individual polypeptides with the mol. wt of the intact molecule indicates that the native molecule contains two 120‐kd subunits and two 62‐kd subunits.