Polyacrolein (PA) microspheres contain reactive aldehyde groups through which ligands containing primary amino groups such as proteins and drugs can be covalently bound in a single step at physiological pH. Antibodies against cyclic-AMP, digoxin and rabbit serum were thus coupled to PA microspheres. The immuno-microspheres were kept in suspension or freeze-dried, with insignificant decrease in their binding capacity. The conjugates were used in the respective radioimmunoassay (RIA) systems to facilitate the separation of the free and the antibody-bound 125I ligands, in comparison with precipitation of Protein A of Staphylococcus aureus. Cyclic-AMP was assayed using PA microspheres coupled either with the primary antibody or with anti-rabbit serum as a secondary antibody, in a buffer system, in chick plasma, in urine and in media in which avian dispersed kidney cells had been stimulated by various agents. The results obtained using the immuno-microspheres and the bacterial separation methods were indistinguishable. Other 125I-ligands, such as digoxin in buffer system or thyroxine and triiodothyronine in chick plasma, were assayed in the picogram range. Owing to the solubility of non crosslinked microspheres conjugates in toluene-based scintillation fluids, both the free and the bound fractions could be counted when using 3H-ligands. Corticosterone was assayed using this technique.