The agglutinating activity of insect serum against vertebrate erythrocytes has been examined for two insect species, the cockroach Periplaneta americana and the locust Schistocerca gregaria. Differences were found between the two insect species, in that cockroach serum agglutinated a wider range of erythrocyte types than did locust serum and the titre of the agglutinating activity of cockroach serum was higher in all cases. The results of attempts to inhibit the agglutinating activity using a variety of sugars and glycoproteins revealed that the combining specificities of the agglutinating molecules of the two species differed. Agglutination of rat erythrocytes by cockroach serum was not inhibited by any of the sugars or glycoproteins tested, whereas several of these compounds, in particular sucrose, partially inhibited the agglutination of rat erythrocytes by locust serum. The significance of these results is discussed in relation to the observation that haemocytes of the cockroach respond to a wider range of transplanted tissues in vivo than do those of the locust.