Electron-microscopic studies were carried out on S and R variants of Brucella suis 1330 and the production of L forms induced with the use of penicillin. No variations were established in the ultrastructure of the initial forms. However, essential differences were observed in the ultrastructure between the latter and the L-transformed Brucellae as well as changes that differentiated the ultrastructure of L forms obtained from S variants from that of L forms obtained from R variants. In both cases the L-transformed Brucella organisms have suffered substantial changes in their electron-microscopic structure. There was no cell wall, the cytoplasm was profoundly altered - chiefly the ribosomal apparatus, which, from a polyribosomal complex located peripherally in the cell, adjacent to the cytoplasmic membrane in the initial forms, has disintegrated into ribosomal units parts of which have often penetrated deeply in the nucleotide mass. Individual ribosomes remained connected with each other through DNA filaments, but there were equally cases in which links of such types were not present. The structure of the cytoplasmic membrane invariably showed three layers. The L forms obtained from R variants occasionally showed the presence of microtubules similar to those established with other microorganisms. Such structures were missing in the L forms obtained from S variants.