Substituted piperazinyliminorifamycins having the following structural formula: ##STR1## wherein Me represents a methyl group and Z represents alkenyl of 3 to 5 carbon atoms, alkynyl of 3 to 5 carbon atoms, 5- or 7-membered cycloalkenyl, cycloalkyl-(C.sub.1 -C.sub.2) alkyl wherein the cycloalkyl moiety is a 3- or 7-membered ring or cycloalkenyl-(C.sub.1 -C.sub.2)alkyl wherein the cycloalkenyl moiety is a 5- or 7-membered ring. The compounds have a broad spectrum antibacterial utility accompanied by a low toxicity.