A novel Ca2+-binding protein (CBP-18) has been identified and purified from bovine brain. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified protein consists of a single band of apparent Mr 18,000 in the presence of Ca2+ or 20,000 in the presence of EGTA. CBP-18 contains one high affinity Ca2+-binding site, measured at 10(-5) M Ca2+ in the presence of 1 mM Mg2+ and 0.1 M K+. The amino acid composition and UV absorption spectrum distinguish CBP-18 from other Ca2+-binding proteins identified in brain. The protein has an extinction coefficient epsilon 1% 279 nm = 4.9 and contains 1 tryptophan/mol, 5 tyrosines/mol, and no trimethyllysine. CBP-18 does not interact with or activate calmodulin-stimulated phosphodiesterase. However, available evidence suggests that CBP-18 binds to other component(s) present in the brain extract in a Ca2+-dependent manner.