Microfilariae have been evaluated as antigen for the indirect immunofluorescent test in the diagnosis of filariasis. Sonicated, unlike whole, microfilariae present no problems in handling on a slide. The cytoplasmic antigen that is exposed by sonication, unlike the sheath or cuticular antigen, reacts with filariasis sera irrespective of whether or not there is a detectable microfilaraemia. The cytoplasmic antigen of microfilariae of various species was marginally superior to Dirofilaria adult worm as antigen for diagnostic immunofluorescence in respect of both sensitivity and specificity. The microfilariae extruded from the uterus of an adult D. immitis were a useful source. But the best results were obtained with the sonicated microfilariae of Brugia pahangi, with which it was possible to utilize both cytoplasmic and sheath antigens simultaneously, giving a positivity rate of 95% for filariasis infecions as a group. This test is thought to be the best available at present for the sero-diagnosis of filariasis, in spite of some lack of specificity. For individual filarial infections a homologous microfilarial antigen is probably the deal.