From the high salt wash of the ribosomes of the yeast Saccharomyces cerevisiae, three protein kinases have been isolated and separated by DEAE-cellulose chromatography. The three kinases differ in their abilities to phosphorylate substrates such as histones (calf thymus), casein, and S. cerevisiae ribosomes; two of the kinases showed increased activity in the presence of cyclic adenosine 3',5'-monophosphate when histones and 40S ribosomal subunits were used as substrates. The protein kinases catalyzed phosphorylation of certain proteins of the 40S and 60S ribosomal subunits, and 80S ribosomes in vitro. Nine proteins of the 80S ribosome, seven proteins of the 40S subunit, and eleven of the 60S subunit were phosphorylated; different proteins were modified to various extents when different kinases were used. We have identified several proteins of 40S and 60S ribosomal subunits which are not available to the kinases in the 80S particles. Ribosomes isolated from S. cerevisiae cells growing in logarithmic phase of growth were found to contain a number of phosphorylated proteins. Studies by two-dimensional polyacrylamide gel electrophoresis indicated that the ribosomal proteins phosphorylated in vivo correspond with those phosphorylated in vitro. The relationship of in vivo phsophorylation of ribosomes to the growth and physiology of S. cerevisiae is not known.