The solution properties of five samples of human immunoglobulin A. (IgA) were investigated with covalent and hydrophobic fluorescence probes. The immunoglobulins included a secretory IgA and four myeloma proteins of both IgAl and IgA2 subclasses in the monomeric and dimeric forms. The probe 8-anilinonaphthalene-l-sulfonate (ANS) was found to bind to both monomeric and dimeric IgA with comparable affinity. Pyrenesulfonyl chloride covalently linked to the proteins exhibited multiexponential decays. The decay of ANS complexed to the same proteins showed similar multiple exponential character. The rotational motions of the immunoglobulins were investigated by the nanosecond fluorescence anisotropy decay method. The decay of both probes attached to these proteins was characterized by a fast component followed by a slow component. The rapid component was in the range 14-26 ns for the covalent conjugates and 26-41 ns for the ANS complexes. These results are interpreted in terms of a segmental motion arising from a mass in the range 60000-100000 daltons. If the decrease in the anisotropy value at long times is taken as a measure of restricted diffusion of the mobile fragment, the half-angle of a cone within which the fragment traverses may provide a qualitative measure of the extent of flexibility. By this criterion, monomeric and dimeric IgA's of the same subclass appear to be qualitatively similar in flexibility. © 1981, American Chemical Society. All rights reserved.