Four ribosomal proteins of Mr 13 kDa, 15 kDa, 19 kDa and 38 kDa were identified as phosphorylation substrates for protein kinases tightly associated with Trichosporon cutaneum ribosomes. It was found that proteins of 13 kDa, 19 kDa and 38 kDa were phosphorylated by multifunctional casein kinase II while the protein of 15 kDa by casein kinase I. Proteins of 13 kDa and 38 kDa were detected in the large subunits while 15 kDa and 19 kDa in the small ribosomal subunits. By using isoelectrofocusing the protein of 13 kDa was resolved into two individual phosphorylated forms. The phosphorylation level of both forms was much higher in ribosomes from the cells collected at the exponential growth phase than in those from the stationary phase. The same phosphoprotein forms were identified in ribosomes from in vitro and in vivo [32P]-labelling experiments.