Colchicine binds to a macromolecule in intact cells, which appears in the soluble fraction after homogenization. The binding is noncovalent and occurs in a single class of binding sites. The binding equilibrium constant and kinetic constants are similar in vivo and in vitro. Bound radioactivity is reisolated and shown to be present in a molecule with the same chromatographic behavior and specific antimitotic activity as colchicine. In vitro assays showed that binding activity correlates with the presence of microtubules. High binding activity was observed in dividing cells, mitotic apparatus, cilia, sperm tails, and brain tissue. Binding to slime mold or purified muscle proteins was low or undetectable. The binding site has a sedimentation constant of 6S and is suggested to be a subunit of microtubules. The binding mechanism involves a noncovalent complex between colchicine and a macromolecule. The radioactivity bound to cells or supernatant extracts was shown to be chemically unchanged. Colchicine inhibits mitosis by interfering with the structure of the mitotic spindle. It also has other effects, such as inducing retraction of axopods, disrupting striations and myotubes in embryonic chick muscle, interfering with cellulose fibril orientation in plant cell walls, suppressing saltatory movement in cultured mammalian cells, causing acute poisoning in warm-blooded animals, and relieving gout symptoms. The results support the hypothesis that the binding site is the subunit protein of microtubules. Colchicine's effects on various systems can be explained by its action on microtubules, which are involved in cell structure and function. The binding activity of colchicine correlates with microtubule abundance rather than mitotic activity or motility. The study concludes that colchicine binds to a macromolecule in intact cells, and the binding is reversible and does not involve chemical modification of colchicine. The binding is described as the simple formation of a noncovalent complex with a single class of binding sites. The binding site is the subunit protein of microtubules.Colchicine binds to a macromolecule in intact cells, which appears in the soluble fraction after homogenization. The binding is noncovalent and occurs in a single class of binding sites. The binding equilibrium constant and kinetic constants are similar in vivo and in vitro. Bound radioactivity is reisolated and shown to be present in a molecule with the same chromatographic behavior and specific antimitotic activity as colchicine. In vitro assays showed that binding activity correlates with the presence of microtubules. High binding activity was observed in dividing cells, mitotic apparatus, cilia, sperm tails, and brain tissue. Binding to slime mold or purified muscle proteins was low or undetectable. The binding site has a sedimentation constant of 6S and is suggested to be a subunit of microtubules. The binding mechanism involves a noncovalent complex between colchicine and a macromolecule. The radioactivity bound to cells or supernatant extracts was shown to be chemically unchanged. Colchicine inhibits mitosis by interfering with the structure of the mitotic spindle. It also has other effects, such as inducing retraction of axopods, disrupting striations and myotubes in embryonic chick muscle, interfering with cellulose fibril orientation in plant cell walls, suppressing saltatory movement in cultured mammalian cells, causing acute poisoning in warm-blooded animals, and relieving gout symptoms. The results support the hypothesis that the binding site is the subunit protein of microtubules. Colchicine's effects on various systems can be explained by its action on microtubules, which are involved in cell structure and function. The binding activity of colchicine correlates with microtubule abundance rather than mitotic activity or motility. The study concludes that colchicine binds to a macromolecule in intact cells, and the binding is reversible and does not involve chemical modification of colchicine. The binding is described as the simple formation of a noncovalent complex with a single class of binding sites. The binding site is the subunit protein of microtubules.