Four monoclonal antibodies (MAbs) were evaluated for specific reactions to various catfish peripheral blood leucocytes and anterior kidney cells. The MAb 9E1 served as a standard and positive control for all test reactions because of its defined reactivity with channel catfish immunoglobulin and immunoglobulin bearing cells. Of the four MAbs, two have been characterized as being specific for a non-immunoglobulin marker on lymphocytes, thus marking T lymphocytes and two were specific for catfish neutrophils. Morphological, flow cytometric and functional analysis of the reactive cells verified these findings. © 1990.