BACKGROUND: C-reactive protein (CRP) and ferritin serum levels represent routine laboratory parameters in the monitoring of renal failure patients. Analysis of CRP, ferritin and other serum proteins can be performed using latex-enhanced or non-latex-enhanced immunoassays. We report on a renal transplant patient with polyclonal IgM hypergammaglobulinaemia having markedly elevated serum CRP and ferritin levels (as detected by latex-enhanced immunoassays) in the absence of clinical signs of an infectious or malignant disorder. METHODS: CRP and ferritin serum levels were determined with various immunoassays with and without latex enhancement. To characterize the causative agent for the elevated CRP and ferritin values, the patient's and a control serum were fractionated by gel filtration on a Sephacryl S-300 column. Serum fractions were subjected to further analysis for reactivity in CRP and ferritin assays. In addition, patient's serum samples were investigated for reactivity with various other latex-based immunoassays (rheumatoid factor, antistreptolysin O, antistreptococcal DNase B). RESULTS: Using latex-enhanced CRP and ferritin immunoassays, markedly elevated serum levels were obtained (CRP 726 mg/l determined by turbidimetry, 398 mg/l determined by nephelometry; ferritin, 20,000 micrograms/l determined by turbidimetry). In contrast, assays without latex enhancement revealed levels within the normal range for both serum proteins (CRP < 5 mg/l, ferritin 52 micrograms/l). The analysis of the patient's serum by gel filtration revealed an interference of the patient's IgM with latex particles used in the CRP and ferritin assays. CONCLUSION: Our study demonstrates that even polyclonal IgM hypergammaglobulinaemia can disturb a large array of latex-enhanced immunoassays used for routine diagnostic procedures. This is of particular interest for the management of allograft recipients in whom monoclonal and polyclonal gammaglobulinaemia are frequently observed. We therefore recommend reanalysis of the respective plasma proteins by latex-free assays in patients with hypergammaglobinaemia showing no clinical signs of an acute infectious disease or malignant disorder.