The complete primary structure of the B subunit of calcineurin (protein phosphatase 2B) has been determined by automated sequence analysis. The protein consists of a single polypeptide chain of 168 residues, relative molecular mass 19200. The structure shows 35% identity with the sequence of calmodulin and 29% with troponin C. Homology is mainly confined to the regions of the four putative Ca2+-binding loops. The results demonstrate that the B subunit is a new member of this family of Ca2+-binding proteins. The N-terminal glycine residue is blocked with the C14-saturated fatty acid myristic acid and the first four residues are very similar to those of the catalytic subunit of cyclic-AMP-dependent protein kinase which also contains a myristoyl blocking group.