PREVIOUS investigators1–4 have described the spontaneous occurrence of hæmolymph nodes in the rat. As a rule, two nodes situated in the vicinity of the kidneys were found to be the most conspicuous representatives of these structures. Their histological features were studied in detail by these workers ; but much uncertainty exists as to the origin and possible function of such nodes, and the same applies to those observed in other species of mammals. It is obvious that useful information might be obtained from experiments aiming at the conversion of ordinary lymph nodes into hæmolymph nodes, and our investigations in this field have led to a number of results, some of which appear to be conclusive. This note describes the effect produced in the lymph nodes of rats by treatment with the carcinogenic hydrocarbon 1: 2: 5: 6-dibenzanthracene, the experiments being suggested by previous observations on rats treated with carcinogenic tar5.