A microorganism Agrobacierium tumefacients as an immobilized cell on a solid support was presented as a new biosorbent for the enrichment of Fe(III), Co(II), Mn(II) and Cr(III) prior to flame atomic absorption spectrometric analysis. Amberlite XAD-4 was used as a support material for column preconcentration. Various parameters such as pH, amount of adsorbent, eluent type and volume, flow rate of sample solution, volume of sample solution and matrix interference effect on the retention of the metal ions have been studied. The optimum pH for the sorption of above mentioned metal ions were about 6, 8, 8 and 6, respectively. The loading capacity of adsorbent for Co(II) and Mn(II) were found to be 29 and 22 [mumol g(-1), respectively. The recoveries of Fe(Ill), Co(111), Mn(l I) and Cr(III), under the optimum conditions were found to he 99 +/- 3, 99 +/- 2, 98 +/- 3 and 98 +/- 3%. respectively, at the 95% confidence level. The limit of detection was 3.6, 3.0, 2.8 and 3.6 ng ml(-1) for Fe(III), Co(II), Mn(II) and Cr (III), respectively, by applying a preconcentration factor of 25. The proposed enrichment method was applied for metal ion determination from water samples, alloy samples, infant foods and certified samples such as whey powder (IAEA-155) and aluminum alloy (NBS SRM 85b). The analytes were determined with a relative error lower than 10% in all samples. (C) 2004 Elsevier B.V. All rights reserved.