The LLC-PK1 mutant cell lines FIB4 and FIB6 are affected in the catalytic (C) subunit of cAMP-dependent protein kinase (cAMP-PK) such that they possess less than 10% parental activity. However, by Western blot analysis they were shown to possess normal levels of C subunit protein. Somatic cell hybrids were derived between mutant and LLC-PK1 cells, and examined for complementation of the cAMP-PK lesion. Codminant expression of mutant and normal alleles was observed, in that somatic cell hybrids between FIB4 and LLC-PK1, and between FIB6 and LLC-PK1 cells, exhibited cAMP-PK activity 60-75% that of LLC-PK1 cells, intermediate between mutant and normal parental cell lines. The cAMP-PK of the FIB6xLLC-PK1 and FIB4xLLC-PK1 hybrids was examined by ion exchange chromatography. In contrast to the FIB6 and FIB4 mutants which lack an active Type I cAMP-PK, the hybrids retained levels of active Type I cAMP-PK greater than 30% that of LLC-PK1, concomitant with the retention of catalytic activity. It was concluded that the loss of Type I kinase in the FIB6 and FIB4 mutants is most likely a consequence of the lesion in the cAMP-PK C subunit. All somatic cell hybrids examined showed levels of cAMP-PK C subunit (as determined by Western blot analysis), and in vivo regulation of cAMP-PK activation (in response to hormonal or nonreceptor-mediated stimulation of adenylate cyclase), completely comparable to those of the parental LLC-PK1 cells. Hence, no aberrant regulation of either cAMP-PK subunit levels or cAMP-PK activities was evident in the somatic cell hybrids. All data were consistent with the hypothesis that FIB4 and FIB6 contain a structural mutation affecting the cAMP-PK catalytic subunit that is expressed phenotypically in the presence of the normal allele.