Certain factors deserve special attention in immunogenetic studies of poikilothermic animals: the effect of environmental temperature, the dosage, route, and time factors of immunization, and the tests for antibodies or immunity employed. Three independent techniques for studies of vertebrate populations are considered. The choice of approach will depend mainly on the requirements for sampling and testing a given species or population. The concurrent use of separate approaches, whenever possible, should augment the criteria available for sub-population differentiations. Considering the evidence as a whole, the "uniqueness of the individual" and the "uniqueness of separate breeding populations" in terms of isoantigenic constitution have emerged as fundamental immunogenetic concepts. This uniqueness manifested by isoantigenic diversity impressively reveals the exceedingly complex genetic structure of vertebrate populations in general.