THE EFFECT OF POLYADENYLIC: polyundylic acid complexes (poly A:U) on the amount of antibody on the surface of various populations of mouse lymphoid cells has been investigated by means of a sensitive measure of such activity-the binding by primed cell populations of beta-galactosidase (betaGZ) as an antigen. The sensitivity derives from the liberation of fluorescein from an artificial substrate, fluorescein-di-beta-galactopyranoside (FDbetaG). After incubation with 100 ng/ml of poly A:U, only 40% of the cells previously showing antigen-binding were still active. The optimum range of activity lay between 0.01-1.0 microg/ml poly A:U. Such cells showed increased RNA and protein synthesis as indicated by [(3)H]uridine and [(14)C]amino acid incorporation. The polynucleotide effect was abolished by incubation of the cells with sodium azide or iodoacetate, but not by puromycin. When the proteins on the cell surface were labeled by (125)I, poly A:U caused their release into the medium. Reports by others that the enhancing effect of polynucleotides on the immune response involves the adenylcyclase system are consistent with the finding reported here that reduction of binding by dibutryl 5'-cyclic monophosphoric acid (cAMP) and poly A:U were parallel in extent, and that theophylline and poly A:U acted synergistically in suboptimal concentrations of each.