Observations of hemocyte aggregation on abiotic surfaces suggested that certain plasmatocytes from larvae of Manduca sexta act as foci for hemocyte aggregation. To establish how these particular plasmatocytes form initial attachments to foreign surfaces, they were cultured separately from other selected populations of hemocytes. While all circulating plasmatocytes immunolabel with anti-β-integrin monoclonal antibody (MAb), only these larger plasmatocytes immunolabel with a MAb to the adhesion protein neuroglian. Neuroglian-negative plasmatocytes and granular cells that have been magnetically segregated from the majority of granular cells adhere to each other but fail to adhere to foreign substrata; by contrast, neuroglian-positive plasmatocytes that segregate with most granular cells adhere firmly to a substratum. Hemocytes form stable aggregates around the large, neuroglian-positive plasmatocytes. However, if neuroglian-positive plasmatocytes are separated from most granular cells, attachment of these plasmatocytes to foreign surfaces is suppressed.