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Cellular populations were studied in lymphoid organs of rainbow trout. Kidney appeared to be the most important organ by the number of cells harvested. A high percentage of peroxidase positive cells was measured in kidney, spleen and blood. The use of cortisone and rabbit anti-thymocyte serum showed differences in ratio of sensitive cells from different organs. In fish treated with steroids, cell depletion was verified in each organ but histologic lesions were observed only in thymus. Rabbit anti-thymocyte serum was cytotoxic for the quasi-totality of thymocytes while it destroyed only a part of kidney, spleen and blood cells. No correlations could be made between steroid sensitive cells and anti-thymocyte serum sensitive cells.

The rainbow trout thymus is a paired organ, organized in 3 adjacent zones. The gland lying on a thick connective tissue layer, is covered by a thin epithelial capsule. Cellular components of thymus are essentially the thymocytes and the epithelial cells. Thymocytes occur mainly in the inner and outer zones but neither cortex nor medulla are clearly delimited. Septa represent a typical epithelial structure associated with thymocytes and blood vessels. Thymus is well vascularized and electron microscopy demonstrates a characteristic relationship between vascular system and lymphoid tissue.