Ornithine decarboxylase (ODC) catalyzes the decarboxylation of ornithine to produce the diamine, putrescine, in the bacterium Escherichia coli. The speC gene encoding ODC has been shown to be subject to transcriptional repression by either putrescine or the cAMP- cAMP receptor protein (CRP) complex. To determine whether these regulatory modes are independent, the expression of ODC was determined by measuring the specific activity of ODC in crude extracts prepared from exponentially grown cultures of wild type ~ coli K-12 as well as in strains unable to synthesize cAMP (cya) and/or CRP (crp). 1-5 mM cAMP repressed ODe activity 22-38% in wild type, 57-66% in the cya strain, and only 7-18% in the ~ strains. 2-10 mM putrescine repressed ODe activity 30-32% in wild type, 48-49% in the cya strain, and 37-38% in the ~ strain. As putrescine repressed ODe activity in the absence of eRP protein (i.e. in a crp strain), putrescine-mediated repression of ODe appears to be independent of the repression of ODC by the cAMP-eRP complex. This conclusion was verified by demonstrating th t .oDC repression by putrescine and cAMP together was additive.