Brain tubulin was labeled in vitro by post-translational incorporation of [¹⁴C]-tyrosine or in vivo by intra-cranial injection of [³H]-leucine. The labeled protein was purified by ion-exchange chromatography. After incubating at 37 °C with a microsomal membrane preparation from rat brain, part of the labeled soluble tubulin became sedimentable at high-speed centrifugation. This was independent of the native configuration of tubulin, the state of tyrosination of the COOH-terminus, or the presence of 100 µM colchicine in the mixture. In addition, the double-labeled tubulin-colchicine complex obtained from the binding of [³H]-colchicine to [¹⁴C]-tyrosinated tubulin, bound to the membrane preparation to the same extent as [¹⁴C]-tyrosinated tubulin. The data show that either tubulin or the complex resulting from its binding to colchicine distributed between the soluble and the membrane fractions when mixed at 37 °C with a microsome preparation. Seemingly, the site for colchicine binding to tubulin needs not to be free for the protein-membrane association.