In the sera of 68 (20.9%) of 325 haemodialysis patients, a specific cold agglutinin, cross-reacting with anti-N, was detected. Each patient showing this anti-N-like antibody used a formaldehyde-sterilized dialyser. By contrast, the antibody was not found in any of the 73 patients using a non-formaldehyde-sterilized dialyser. The results of transfusion experiments indicate an in vivo haemolytic activity of the antibody. In vitro, formaldehyde pretreatment of erythrocytes increased the agglutinability of the cells by the anti-N-like antibody. This effect could be prevented when the MN receptor of the red cells was exposed to neuraminidase. Trace quantities of formaldehyde inevitably enter the patient during dialysis when a formaldehyde-sterilized dialyser is used. It is postulated that this in vivo formaldehyde exposure might interact with the MN receptor of the red cells, rendering it immunogenic, and thus induce the formation of the haemodialysis-associated anti-N-like antibody.