Suspension of isolated adrenal cells from the guinea pig adrenals was prepared using two modifications of collagenase dispersion. The cells separated by percoll density gradient gave 4 bands. The fraction of adrenocorticocytes has buoyant density of 1.03-1.05 g/ml. The lower bands comprise blood cells with density of 1.07-1.10 g/ml. The aldosterone content in the adrenocorticocyte fraction is 1200-1300 pg/10(5) cells. Intensity of labelling of cellular proteins increases only in the adrenocorticocyte fraction in response to a rise in K+ concentration in incubation medium. The Scatchard plot method was used to characterize binding of 125I-ACTH to dispersed adrenocortical cells. Binding association constant (Ka) and binding capacity (Bmax) are 1.3 X 10(8) M-1 and 2.0 X 10(-9) mol/l, respectively. A relation between functional activity of adrenocortical cells, their ultrastructural features and density is discussed.