Abstract: Equine serum and secretions were found to contain a protein which
cross-reacted with an antiserum against human IgA, but not with antisera against
any other human immunoglobulin. The physicochemical properties of equint
IgA resembled those of human IgA. IgA was found to be the immunoglobulin
having the highest secretion vs serum concentration ratio in equine lacteal and salivary
secretions, and to be the protein produced by the majority of immunoglobulin-
containing cells in the lamina propria of the equine intestine.