Although serum and/or secretory IgA has been well defined in the human as well as in several laboratory animals including mice, rabbits and dogs (1, 2), this immunoglobulin has not been adequately identified and characterized in the rat. Several reports have appeared in which a rat serum immunoglobulin of relatively slow mobility was referred to as IgA (3–5) or γ1 (6). This identification was made, for the most part, on the basis of electrophoretic mobility. The purpose of this communication is to demonstrate that the mobility of rat serum IgA is faster than that of the presently known rat immunoglobulins, and that some of the previous papers dealing with rat IgA refer to a serum immunoglobulin which may in fact be homologous to mouse IgG1 (IgF). Of the many possible criteria which can be used in identifying a specific immunoglobulin class within a species, the most precise would appear to be a comparison of amino acid sequences between known immunoglobulins and the protein being studied.