Growth inhibition due to iron deprivation of 118 strains of staphylococci, enterobacteriaceae, Streptococcus faecalis and Bacteroides species was studied in vitro. Apo-transferrin (Tr), apo-lactoferrin (Lf) and a synthetic iron chelator (EDDA) were used as iron binding agents. S. epidermidis was more inhibited than S. aureus. The majority of the enterobacteriaceae was inhibited by the highest concentrations of EDDA, Tr and Lf. Most bacteroides strains were inhibited by EDDA and all were inhibited by Tr and Lf at physiological concentrations. Production of bacterial iron chelators could be demonstrated in all strains growing at iron deprivation except for Streptococcus faecalis. In 8 out of 11 strains of Bacteroides cytochromes of type b and c could be demonstrated. In one strains iron deprivation resulted in disappearance of the cytochrome and a reduction of the growth yield. In Bacteroides the production of iron-chelators could not be related to virulence.