Immune channel catfish lymphocytes were recovered from peripheral blood on an isotonic (250 mOsm) separation medium consisting of Ficoll‐Hypaque adjusted to a specific gravity of 1·065–1·070 g ml−1. At least two types of cells similar to T and B lymphocytes of higher vertebrates were observed. One type of lymphocyte, referred to as “small lymphocyte” possessed surface erythrocyte receptors and appeared to be analogous to mammalian T cells. A second type of lymphocyte analogous to mammalian B cell possessed surface immunoglobulins and was capable of cytoadherence withAeromonas hydr