In spring and autumn, the lymphoid complex of the colubrid snake, Psammophis schokari is well developed and the humoral response to intraperitoneally administered rat erythrocytes (RRBC), human serum albumin (HSA), or polyvinyl pyrrolidone (PVP) is quick and vigorous. In contrast, in summer and winter, lymphoid elements of the thymus, spleen, and part of the gut-associated lymphoid tissue are highly depleted. Humoral reactivity to RRBC and HSA is poor in summer presumably due to the deleterious effect of seasonal factors on the thymus and thymus-derived helper lymphocytes. This assumption is supported by the fact that, in summer, P. schokari responds powerfully to the T-independent antigen PVP. Humoral response to RRBC, HSA, and PVP is, however, weak in winter suggesting that a further mechanism of immunosuppression is involved.