Cytochemical enzyme staining of fish lymphocytes separated on a Percoll gradient

Brown trout (Satmo trutta L) lymphocytes were shown to separate into two fractions on a Percoll discontinuous gradient, with 53% of the cells in the 1.07gl‐1 fraction. The cells from the two fractions showed equal enzyme activity when stained for acid esterase and acid phosphatase. About 70% of the lymphocytes gave a positive enzyme reaction, which if the reaction is comparable with mammalian lymphocyte cytochemistry would indicate they were T‐lymphocytes. There appears to be increasing evidence among fish for the existence of T‐ and B‐lymphocytes, and cytochemical staining could prove a comparatively convenient method for their demonstration.