Tyrocidine, a peptide antibiotic produced by Bacillus brevis (ATCC 8185), relaxes superhelical DNA in a biphasic manner and induces ‘packaging’ of the DNA at higher concentrations. This was concluded from studies using the sensitive 4,5′,8-trimethylpsoralen photobinding technique [Sinden, R. R., Carlson, J. O. & Pettijohn, D.-E. (1980) Cell 21, 773–783]. Relaxed DNA is not affected by tyrocidine wheras linearized molecules become packaged. The linear gramicidin synthesized by the same strain reverses the tyrocidine-induced relaxation as well as the packaging, an observation which might be of biological relevance.