The cyclic lipopeptide globomycin, a specific inhibitor of signal-peptidase II (Lsp A), proved toxic for the mollicute Spiroplasma melliferum with a minimal inhibitory concentration (MIG) in the range 6.25-12.5 mu M, about one order of magnitude higher (that is, less efficient) than bee-venom mellitin. SDS-PAGE analysis of cell proteins followed by immunolabeling (''Western blotting'') and by crossed immunoelectrophoresis demonstrated that the cleavage of the prespiralin leader peptide was prevented by globomycin. Cell fractionation experiments showed that prespiralin was membrane bound and did not accumulate in the cytoplasm or in the culture medium. Furthermore, the use of the potential-sensitive fluorescent dye 3,3'-dipropyl-2,2'-thiadicarbocyanine iodide (diS-C-3-[5]) revealed Chat, in contrast to valinomycin and mellitin, globomycin up to 30 mu M has no effect on the electrical transmembrane potential of S. melliferum. This indicates that the toxicity of globomycin for spiroplasma cells is mainly if not exclusively owing to the inhibition of spiralin processing. Added to previously published data, these results suggest that spiralin and probably other lipoproteins of mollicutes are acylated and membrane targeted by a mechanism involving notably the processing of the prelipoprotein precursor by a type II, globomycin-sensitive signal peptidase.