Studies were performed on two anti-N sera from patients who had formed anti-N antibodies during their period of haemodialysis. No specific inhibition of these antibodies was obtained by extracts prepared from a new and a used dialysis unit, and it was concluded that there was no evidence of a compound in the units which would stimulate production of the anti-N. The specificity of the anti-N's was shown to be against a precursor of MN antigens, since all activity in the sera could be removed by neuraminidase-treated M cells, which expressed MN precursor. Since the anti-N's would react with untreated N cells only at 4 degrees C, but with formaldehyde-treated cells at 4, 25 and 37 degrees C, it was concluded that the specificity of the anti-N's was directed against an antigen developed by red cells after formaldehyde treatment. It is proposed that the production of the anti-N antibodies is stimulated by formaldehyde-treated red cells passing into the patient during re-use of a dialysis unit sterilized with formaldehyde.