A quantifiable assay is described in which the lateral cilia of the sea musselMytilus edulisare used to examine the effects of serum on ciliary function.Human sera and sera from a variety of animals have been found to induce a change in ciliary metachrony. This alteration in activity appears to be brought about by the agglutination of adjacent cilia into blocks, with the effect of increasing the wavelength; the beat frequency remains steady. Parallel studies with the rabbit cilia assay have shown the agglutination of tracheal cilia into clumps by adult bovine serum. The dilution of adult bovine serum lengthens the time before dyskinesia is first observed in both theMytilusand rabbit assays. Fetal bovine or human cord sera do not cause ciliary dyskinesia or ciliary agglutination.The evidence presented shows that the observed ciliary dyskinesia can be induced by sera from a variety of animals and is not specific to cystic fibrosis; it also suggests that the serum immunoglobulin fraction is responsible for these effects—initial studies implicate IgM.