Abstract When the plasmacytoma cell line MPC-11 was grown at high cell density in suspension culture (limited feeding) an approximately stoichiometric ratio between ribosomal 60S and 40S subunits was found in the cytoplasm. When grown in suspension culture at low cell density (well-fed cells) the ratio of 40–60S ribosomal subunits was higher than stoichiometric. A transfer from high cell density (poor growth conditions) to low cell density (good growth conditions) by addition of fresh medium strongly favoured accumulation of native 40S subunits in the cytoplasm compared with the accumulation of native 60S subunits. Both absorbance measurements and rRNA labelling experiments suggested the presence of an excess of 40S subunits under these conditions. The ratio between the ribosomal subunits in the cytoplasm of this transformed cell line, therefore, is strongly influenced by the growth conditions.