Two different lectins (termed BnA-I and BnA-II) with distinct carbohydrate specificities were identified and subsequently isolated from the marine bryozoan Bugula neritina. BnA-I hemagglutinating activity was inhibited by N-acetylated hexosamines, their polymers, and glycoproteins rich in these moieties. BnA-II-induced hemagglutination was not blocked by any simple sugars but could be inhibited by several complex glycoproteins (e.g., thyroglobulin and orosomucoid). Both lectins required the presence of Ca(+)+ for reactivity and were purified by affinity chromatographic procedures. Purified BnA-I was determined to have a native molecular weight of 240 Kd and appeared to be a hexameric homopolymer while BnA-II was shown to be a 65-70 Kd monomer. Both lectins showed seasonality in expression, BnA-I appearing in animal extracts prepared in the spring and fall while BnA-II was expressed only during the summer and winter.