Ribosomes and subunits from Acanthamoeba castellanii were purified. The pattern of ribosomal proteins was analysed by two-dimensional gel electrophoresis. The small subunit of cytoplasmic ribosomes contains 25 and the large 40 proteins. After incubation of exponentially growing cells with ortho[33P]phosphate for 1 h, one basic protein of the small subunit was radiolabelled. This phosphorylated protein has a molecular weight of about 45000 as estimated by dodecylsulfate/polyacrylamide gel electrophoresis. The level of phosphorylated ribosomal protein as estimated by staining of proteins separated by two-dimensional gel electrophoresis changes in response to growth conditions. The maximum level occurs after adding fresh nutrient medium to cells which have been allowed to grow to high density, whereas ribosomes of exponentially growing cells contain low amounts. Transfer of exponentially growing cells into a non-nutrient medium induces the formation of cysts containing ribosomes almost devoid of phosphorylated protein.