A partial cDNA was isolated that encoded a protein kinase, termed rac (related to the A and C kinases). This cDNA was subsequently used to screen libraries derived from the human cell lines MCF-7 and WI38 and led to the isolation of full-length cDNA clones. DNA sequence analysis identified an open reading frame of 1440 base pairs encoding a protein of 480 amino acids (Mr, 55,716). This result was supported by the synthesis of a Mr 58,000 protein in an in vitro translation system that used RNA transcribed from cloned cDNAs with SP6 RNA polymerase. The predicted protein contains consensus sequences characteristic of a protein kinase catalytic domain and shows 73% and 68% similarity to protein kinase C and the cAMP-dependent protein kinase, respectively. Northern (RNA) analysis revealed a single mRNA transcript of 3.2 kilobases that varied up to 300-fold between different cell lines. Specific antisera directed towards the carboxyl terminal of the rac protein kinase were prepared and used to identify that phosphorylated several substrates in immunoprecipitates prepared with the rac-specific antisera.