A cyclic AMP-dependent protein kinase activity from epimastigote forms of Trypanosoma cruzi was characterized. Cytosolic extracts were chromatographed on DEAE-cellulose columns, giving two peaks of kinase activity, which were eluted at 0.15 M- and 0.32 M-NaCl respectively. The second activity peak was stimulated by nanomolar concentrations of cyclic AMP. In addition, a cyclic AMP-binding protein co-eluted with the second kinase activity peak. Cyclic AMP-dependent protein kinase activity was further purified by gel filtration, affinity chromatography on histone-agarose and cyclic AMP-agarose, as well as by chromatography on CM-Sephadex. The enzyme ('holoenzyme') could be partially dissociated into two different components: 'catalytic' and 'regulatory'. The 'regulatory' component had specific binding for cyclic AMP, and it inhibited phosphotransferase activity of the homologous 'catalytic component' or of the 'catalytic subunit' from bovine heart. Cyclic AMP reversed these inhibitions. A 'holoenzyme preparation' was phosphorylated in the absence of exogenous phosphate acceptor and analysed by polyacrylamide-gel electrophoresis. A 56 kDa band was phosphorylated. The same preparation was analysed by Western blotting, by using polyclonal antibodies to the regulatory subunits of protein kinases type I or II. Both antibodies reacted with the 56 kDa band.