Microorganisms require iron for their growth and usually compete with their host for available iron from the system. Iron supplementation to host causes an increase of available iron both to host and to potential microbial invaders and favours the latter more than the former as the bacteria release siderophores which are responsible for iron transport mechanism. In view of this observation a study was done to deal with the distribution of storage and injected iron given as an overload within a physiological pool, taking mice as the host, with a correlation to its utilization by Agrobacterium tumefaciens and with bacterial growth and multiplication. The results obtained help in understanding the host - parasite relationships, regarding bacterial virulence and infection and the growth-promoting effect of iron, as iron promoted the development and progression of serum-exposed A. tumefaciens in mice. © 1988 Institute of Microbiology, Academy of Sciences of the Czech Republic.