Some medical centers re-use dialysis units sterilized with formaldehyde. In a study of 239 cases, 14 "anti-N" antibodies were found only among the 59 patients of the medical centers which re-use dialysis units. The action of formol seems to be confirmed by the presence of "anti-N" in 2 patients who had undergone prosthesis several times, but not dialysis. For these prostheses, a bone cement, sterilized with formol, was used. These "anti-N" are very often associated with cold autoagglutinins, and appear regardless of the patient's MN group. The action of formaldehyde suggests the following hypotheses:--antigenic modification;--disturbances in the immune response mechanisms;--a combination of the two. In the first hypothesis: the action of formol discovered since a long time on red cells. In the second hypothesis: the existence of auto-agglutinins only among the 14 hemo-dialysis patients with anti-N antibodies.