ABSTRACT. To obtain biologically active substances that are useful in the study of cell structures and functions, we examined several biological activities of an extract of silkworm faeces with hot buffer. Wedetected a lectinlike substance in the extract and purified it. Whenthe extract of silkworm faeces was added to a culture of quail myoblasts transformed with temperaturesensitive Rous sarcoma virus (QM-RSVcells), the plated cells which were cultured at 35.5°C (the permissive temperature for RSVthat suppresses myogenic differentiation) became detached from the dishes and the cells in suspension aggregated. However, when the same amount of extract was added to plated QM-RSV cells cultured at 41°C, which is a nonpermissive temperature for RSV, the cells did not become detached. Other kinds of plated cells examined also did not become detached. Unlike plated cells, nonplated cells of all kinds aggregate in suspensionuponthe addition of the extract. This active substance was purified by monitoring its induction of cell aggregation. The purified substance was found to be a lectinlike glycoprotein with an apparent molecular mass of about 60 kDa. Further studies showed that the binding sites of this glycoprotein are sugar chains on the cell surface and that mannoseis an epitope.