SUMMARY An immunological method based on the specificity of delayed-type skin reactions on guinea-pigs has been used for comparing 17 Nocardia strains. By this method, 4 isolates labelled Nocardia farcinica, four labelled N. asteroides and I strain labelled N. blackwellii could be grouped together with the type strain of N. farcinica. Three other strains labelled N. farcinica were clearly distinct from this group of 10 strains. A strain of N. asteroides used previously as reference strain for this species was clearly distinct from the N. farcinica strains, and this applied also to a recently proposed reference strain of N. asteroides. A strain of N. brasiliensis was clearly distinct from all the other strains, while the results obtained with a strain received without species designation were inconclusive. On this basis, the authors consider that all the 10 strains, which form a homogeneous group, should be classi- fied as iV. farcinica. The 3 other strains labelled N. farcinica should be classified into 2 other species. Nocardia asteroides would seem to be a separate species.