Emerimicins II, III and IV are new antibiotics produced by Emericellopsis microspora when trans-4-n-propyl-L-proline (propyl proline) is added in the fermentation medium. Emerimicins are similar to antiamoebin and stilbellin. However, all emerimicins have been differentiated from the latter antibiotics by comparison of tic behavior and aminoacid composition. Propyl proline appears to induce the production of the new antibiotics since the aminoacid was not incorporated into the molecules of emerimicins II, III, or IV. In the course of work related to the effects of environmental changes on antibiotic production, we observed that a microorganism identified as Emericellopsis microspora, grown in a complex medium, produced several antibiotics only when trans-4-n-propyl-L-proline was added to the medium. Three of these antibiotics named emerimicins II, III and IV have been isolated crystalline and characterized. The production, isolation and characterization of these antibiotics is the subject of the present communication.