Anti N(DP) has been detected in the sera of 38 of 430 patients on regular hemodialysis at the Regional Kidney Disease Program in Minneapolis. It developed in these patients from 7 to 58 months after commencement of dialysis. Bacterial infection appeared temporally related to the development of anti N(DP) in 12 patients. Hemolytic episodes, possibly related to anti N(DP), occurred in 11. Fifty five percent of the patients never reused dialyzers. The antibody preceded the insertion of a bovine graft in seven. We postulate that anti N(DP) is recognizing an antigenic site similar to that recognized by Vicia graminea lectin, and that this site might become immunogenic by alteration of M and N antigens on red blood cell surfaces. Though formaldehyde might be involved in this alteration, dialysis membrane reuse does not seem to be required for the formation of anti N(DP).