We report that a human neoplastic B cell line (Reh) contains cAMP-dependent protein kinase (cAK) type I (cAKI), but is practically devoid of cAK type II (cAKII). However, these cells contain a novel cAKI isozyme consisting of an RI alpha-RI beta heterodimer in association with phosphotransferase activity (RI alpha RI beta C2) eluting from DEAE-cellulose columns at a salt concentration characteristic of a cAKII. Immunoprecipitation of 8-azido-[32P]cAMP-labeled extracts and DEAE fractions employing specific antibodies directed against RI alpha and RI beta clearly demonstrated the presence of RI alpha-RI beta heterodimers. RI alpha was precipitated with RI beta antiserum and vice versa. Furthermore, disruption of disulfide bridges by reduction-alkylation abolished this coimmunoprecipitation. In addition, formation of heterodimeric complexes of RI alpha and RI beta could be demonstrated in vitro using recombinant RI proteins. Finally, the presence of low levels of RI alpha-RI beta heterodimers could also be demonstrated in human peripheral blood T lymphocytes. RI alpha-RI beta heterodimers complexed with the catalytic subunit represent a novel isozyme of cAKI (RI alpha RI beta C2), which enhances the possibilities for diversification of cAMP-mediated effects.