Two cDNA clones coding for the human protein kinase C-delta (PKC-delta) were fortuitously isolated during the process of screening a human library for a cDNA clone of an unrelated protein, the nucleolar protein fibrillarin. The two human homologues have about 88% nucleotide sequence identity to the rat and mouse PKC-delta cDNA clones. A comparison of the predicted amino acid sequences of the two human PKC-delta clones with the rat and mouse homologues indicated a greater degree of sequence divergence (89-90% homology) compared to the high degree of sequence conservation observed with other human PKC family members and their mammalian counterparts. Expression of the clones in the baculovirus insect-cell expression system indicated that both proteins exhibited phorbol ester binding activity, and were dependent upon phosphatidylserine and diacylglycerol for maximal activation. Further characterization of the properties of the human PKC-delta revealed substrate and lipid dependencies distinct from other members of the protein kinase C family; including PKC-deltas isolated from other species. The dissimilarities in the predicted amino acid sequences between the human and other mammalian species could account in part for some of these observed biochemical differences.