A new effective insoluble support useful for cell fractionation based on agarose-polyacrolein microsphere beads (APAMB) of diameters 150--250 micrometers has been developed. The synthesized polyacrolein (PA) microspheres, of average diameter 0.2 micrometer, are provided with reactive aldehyde groups through which various ligands containing primary amino groups are bound covalently in a single step at physiological pH. Antibodies coupled to the microspheres are very effective for labelling of cell surface receptors on human red blood cells and mouse lymphoid cells. APAMB were obtained by encapsulating the PA microspheres with agarose. Antibodies and lectins bound to the APAMB serve to construct affinity columns for the separation of red blood cells and murine lymphocyte subpopulations. Anti-human red blood cell antibodies coupled to anti-immunoglobulin APAMB are effective in separating human from turkey red blood cells, whereas either anti-Thy 1.2 anti-immunoglobulin antibodies or soybean agglutinin coupled to APAMB have proved useful for the separation of T and B cells from heterogeneous population of spleen cells. The separation procedure is simple, rapid and effective. The viability of the fractionated cells is unaffected by the procedure and the recovery of the cells is high: between 80% and 100%.