The primary structure of human platelet profilin was determined by aligning the sequences of its tryptic peptides to the previously determined calf spleen profilin sequence [(1979) FEBS Lett. 101, 161-165]. Comparison of the peptide fingerprints of the two proteins suggested a higher homology than that found by direct sequence comparison. We therefore reinvestigated the sequences of the peptides from calf spleen profilin. We identified four incorrect charge assignments and a deletion of three residues. The similarity between the two vertebrate profilins amounts to 95%.