In winter, the lizard, Scincus scincus, fails to form humoral antibodies to parenterally introduced rat erythrocytes. The response is slow and feeble in spring whereas, it is quick and vigorous in summer and autumn. In winter, the thymus of Scincus is involuted, white pulp of spleen highly depleted while, gut-associated lymphoid tissue is rather developed. By the beginning of spring, lymphoid organs start to regenerate. By the end of spring through midfall, the thymus presents a rich lymphoepithelial organization; splenic lymphoid aggregates are so developed that they become confluent and gut aggregates increase in number and size. The data suggest that, the kinetics and magnitude of the lizard's humoral response to foreign erythrocytes, correlate with the state of lymphoid tissue development which, in turn, is affected by seasonal variations.