The windowpane flounder, Lophopsetta maculata, was found to have proteins in the body mucus which agglutinate mouse leukemia cells, L5784Y but not L1210. They also agglutinate rabbit and mouse erythrocytes, a marine yeast and a bacterium, and have weak activity against mouse sarcoma 180 cells, human B, guinea pig, and horse erythrocytes. The hemagglutinating activity was not affected by the treatment with 2-mercaptoethanol, trypsin, or pronase, but was inhibited by a high concentration of N-acetylneuraminic acid. The major active component was purified and found to be a protein having a molecular weight of 68 000 which dissociates into subunits of equal size (16 000). Isoelectrofocusing gave two sharp bands, close together, at p I 4.7 ± 0.1. The protein contains high amounts of aspartic acid, glutamic acid and glycine, and very little histidine and half-cystine.