Transcriptional regulation of eukaryotic genes by cyclic AMP requires a cAMP-dependent protein kinase (A kinase). Two hypotheses have been proposed to explain how the holoenzyme of the A kinase induces transcription. The regulatory subunits of the A kinase, which bind cAMP and DNA, and have amino-acid homology with the Escherichia coli catabolite activator protein could directly stimulate gene expression. Alternatively, phosphorylation by the catalytic subunits could induce transcription by activating proteins involved in gene transcription. To distinguish between these models, we microinjected purified preparations of the catalytic and regulatory subunits of A kinase into tissue culture cells and monitored expression of a stably integrated fusion gene containing a cAMP-responsive human promoter fused to a bacterial reporter gene, or of the endogenous c-fos gene. The catalytic subunit stimulated expression of these genes, whereas the regulatory subunit did not. These results indicate that the catalytic subunit of A kinase is sufficient to induce expression of two cAMP-responsive genes, without increasing levels of cAMP.