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Two kinds of tubulin (alpha and beta) have been described in microtubules from many different systems. In this study a discontinuous acrylamide-gel system containing sodium dodecyl sulfate was used to separate milligram quantities of alpha- and beta-tubulin from microtubules of chick-embryo brain and from outer doublets of sea-urchin sperm. The isolated tubulins were characterized by peptide mapping and automated sequencing of the first 25 NH(2)-terminal amino acids. Our results show that alpha- and beta-tubulin are related but distinctly different proteins and that each one has been highly conserved in the course of evolution.