Rat brain tubulin purified by colchicine-agarose affinity chromatography contains protein kinase activity. The kinase activity can be separated completely from tubulin by chromatography on casein columns and is not subsequently retained by colchicine affinity columns. Protein kinase activity associated with purified tubulin does not correlate with the total content of protein kinase activity in brain homogenates, since microtubules isolated from 48 000g fetal brain supernatants contain twice as much protein kinase activity than adult microtubules, although the total protein kinase activity is twice as high in the 48 000g adult supernatant. The protein kinase of tubulin preparations, while corresponding to a different molecule than tubulin, is probably not simply the result of contamination. These observations are interpreted in terms of specific associations between protein kinase and tubulin complexes. The protein kinase-tubulin association may be an important determinant in the regulation of tubulin function. Fetal tubulin polymerizes twice as well as adult tubulin in the absence of glycerol at the same tubulin concentration. The preferred substrate for the protein kinase either in vivo or in vitro (pH 7.4, 37 degrees C) is a specific high-molecular-weight protein, distinct from tubulin, which copurifies with tubulin through different kinds of isolation procedures (i.e., colchicine affinity chromatography and ammonium sulfate precipitation followed by diethylaminoethyl-cellulose chromatography). The tubulin-associated protein kinase is completely dependent on cyclic adenosine monophosphate (Km=10(-7)M), as demonstrated by the complete suppression of activity upon addition of the protein kinase modulator, a well-known specific inhibitor of cAMP-dependent protein kinases