A universal chemical assay (Schwyn and Neilands , 1987) was used to detect the production of siderophore in a range of rhizobacterial strains. Siderophore production was found to be strain specific In Rhizobium. The product ion of siderophore by Rhizobium meliloti 220-5 was examined in detail. Using the universal assay to test samples taken during growth of the bacterium in an iron deficient medium, it was established that product ion of the siderophore commenced at the onset of exponent ial growth. The cell free supernatant of R.meliloti 220-5 grown under iron deficient conditions and shown to contain a siderophore, was tested for the presence of catechol or hydroxamate groups. Neither group was detec ted.The cell free supernatant of R.meliloti 220-5 grown under iron deficient conditions and shown to contain a siderophore, was tested for the presence of catechol or hydroxamate groups. Neither group was detected.The cell free supernatant of R.meliloti 220-5 grown under iron deficient conditions and shown to contain a siderophore, was tested for the presence of catechol or hydroxamate groups. Neither group was detected. Using a phenol/chloroform extraction procedure the siderophore was isolated from low iron supernatants and purifie d by ion-exchange chromatography. IR spectra confirmed that the siderophore was neither catecholnor hydroxamate in nature. This indicates that it belongs to a third class of siderophores typified by rhizo bactin from R.meliloti DM4 (Smith and Neilands, 1984). Mutants o f R.meliloti 220-5 and R.meliloti 2011 d e f e c t iv e in the production of siderophore were isolated following transposon mutagenesis with Tn5-mob. The genomic sequence conta ining the transposon from R.meliloti 220-5-1 was cloned into pUC19 and used to screen a cosmid bank of R.meliloti 2011. Two cosmids hybridising were investigated. One cosmid, cosmid 3, complemented the mutation in R.meliloti 220-5-1. When the other mutants were tested for complementation by cosmid 3, the complementation pattern in dicated that cosmid 3 harbours more than one gene involved in the siderophore b io s yn thesis. The number of siderophore genes carried on this cosmid remains to be determined. SDS-polyacrylamide gelelec trophoresis of outer membrane proteins prepared from iron deficient cultures identified the presence, in R.meliloti 220-5, of two low iron induced proteins of 72,000 and 78,000 daltons. These induced prote ins were cut from prepa rative SDS-PAGE gels and used as antigens to raise polyclonal antibodies. Serum collected from the injectedrat was tested for the antibody by Western blot analysis.