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Ca²⁺‐dependent protein kinase (CDPK‐1) was purified from maize seedlings, and its substrate specificity studied using a set of synthetic peptides derived from the phosphorylatable sequence RVLSRLHS¹⁵VRER of maize sucrose synthase 2. The decapeptide LARLHSVRER was found to be efficiently phosphorylated as a minimal substrate. The same set of peptides were found to be phosphorylated by mammalian protein kinase Cβ (PKC), but showed low reactivity with protein kinase A (PKA). Proceeding from the sequence LARLHSVRER, a series of cellulose‐membrane‐attached peptides of systematically modified structure was synthesised. These peptides had hydrophobic (Ala, Leu) and ionic (Arg, Glu) amino acids substituted in each position. The phosphorylation of these substrates by CDPK‐1 was measured and the substrate specificity of the maize protein kinase characterised by the consensus sequence …