SummaryCats infected with Brugia pahangi were immunosuppressed and the development of lymphatic pathology, as measured xeroradiographically by dilatation of the vessels, in these animals was compared with that of normally infected cats. Individual cats showed wide variations in response. Niridazole and antilymphocyte serum both reduced dilatation, niridazole being particularly effective in the early stages of infection. When silica, used as an inflammatory agent, was injected subcutaneously in conjunction with worm infection lymphatic dilatation developed, this being potentiated in degree and rate of development as compared with control animals. The possible role of a non‐filarial inflammatory response in the development of lymphatic dilatation is discusse