Different classes of immunoglobulins (immunoglobulin M [IgM], IgG, and secretory IgA) were purified from pooled serum and milk samples of convalescent cholera patients by gel filtration and immunoadsorbent techniques. The purity of these preparations was established by immunodiffusion and the enzyme-linked immunosorbent assay, using class-specific antisera. The biological properties of antibodies present in these crude and purified immunoglobulin preparations were evaluated by tests related to cholera. Purified human IgM and IgG exhibited both agglutinating and vibriocidal properties. On the other hand, human secretory IgA was not vibriocidal (even in the presence of lysozyme), although it showed agglutinating properties. Both IgG and secretory IgA could effectively neutralize cholera toxin action in vivo, whereas such activity was virtually absent in IgM. The toxin-neutralizing capacity of IgG was, however, higher than that of secretory IgA. All three classes of human antibodies could significantly inhibit the adherence of Vibrio cholerae to intestinal slices in vitro. These results are discussed in relation to the protective immune mechanism during cholera infection.