We have reported that serum antibodies of the gar, Lepisosteus platyrhincus, a fresh water holostean fish, are restricted to the macroglobulin fraction (1). In studies to be reported we will present evidence that this species forms only one class of immunoglobulin, i.e., IgM (Bradshaw, C. M., Clem, L. W., and Sigel, M. M., to be published). In more recent work we have detected IgM antibody in mucus secretions of the gar. These findings are presented in the present paper.Materials and Methods. Animals. The collection, maintenance and bleeding of the gar, Lepisosteus platyrhincus, were as described previously ( 1 ) .Fish were immunized with 1 ml of sheep erythrocytes (ShE) prepared in 0.15 M phosphate buffered saline (PBS), pH 7.2, and standardized spectrophotometrically to 1 x 1 O9 cells/ml, administered intramuscularly into two sites on either side of the backbone.Mucus preparation. Mucus was collected by wiping seven fish with filter paper. 150 ml of Tris-HC1 buffer, 0.14 M NaCl, pH 7.4, was added and allowed to stand overnight a t 4'. The material was then centrifuged a t 1500 rpm for 20 min and the supernatant was concentrated by positive pressure dialysis (1 atm of pressure) at 4".Antibody assay. Antibodies were detected by agglutination of ShE or other erythrocytes using the microtiter assay system (Cooke Eng. Co., Alexandria, Va.). The standard diluent for all antibody assays was PBS. Erythrocytes were washed three times with PBS and standardized spectrophotometrically to 1 x 1@ cellsJm1 for titration. Serum or mucus samples were serially diluted in a