Exposure of E. coli B/r to nitrofurazone strongly inhibits the synthesis of all classes of RNA and both ribosomal sub-units. Polysome formation is likewise inhibited. However, in E. coli nfr-207 a mutant of B/r which lacks nitrofurazone-reductase I, the synthesis of RNA, ribosomal sub-units and formation of polysomes are not significantly affected. This result implies that a reduced metabolite of the drug rather than the drug itself is the active agent. The ability of ribosomes isolated from nitrofurazone-treated E. coli B/r to carry out poly-U directed polyphenylalanine synthesis was lower than that of ribosomes from untreated cells. 14C from labelled nitrofurazone was found to bind to ribosomal sub-units.