Actin was purified from calf brains by chromatography on DEAE-Sephadex and hydroxylapatite. The protein was then subjected to amino acid sequence analysis by isolating and sequencing its cyanogen bromide peptides. CB-1, 3, 4, 5, 6, 9, 10, and 12 correspond to equivalent segments of rabbit skeletal muscle actin, while subsitutions involving methionines give rise to some new peptides. The region that corresponds to CB-13 in muscle actin becomes two peptides in the brain protein because of a Leu leads to Met replacement at position 16, while Met leads to Leu substitutions at positions 176 and 298 give rise to two larger peptides, CB-15 + 7 and CB-8 + 2, which correspond to muscle actin CB-15 fused with CB-7 and CB-8 fused with CB-2, respectively. The peptides that have been isolated from brain actin contain 267 of the 374 residues in actin, of which 157 have been unequivocally identified. When the data are compared with those for rabbit skeletal muscle actin, 11 replacements are seen; thus the two actins differ at about 7% of the positions examined.