In order to determine the degree and significance of red cell antibody production by dialysis patients, two groups of patients were studied retrospectively. One hundred and five randomly selected dialysis patients (Group I) receiving a total of 1074 units of blood were reviewed, as were 38 patients who were given frozen-thawed red cells because of intractable nonhemolytic febrile reactions following transfusions of red cells stored conventionally (Group II). Eleven patients in Group I produced alloantibodies: nine after the start of dialysis. Of these, two patients had antibodies that were judged clinically insignificant. Eleven of 38 patients in Group II had red cell alloantibodies at the time of study, four of which were judged clinically significant. The direct antiglobulin test (DAT) was positive in 14 patients in Group I and eight patients in Group II for an overall percentage of 15. The cause of the positive DAT was determined in eluates for 26 percent of the cases studied. Clinically significant antibodies were produced by 6.7 percent of randomly selected dialysis patients (Group I). This incidence is lower than other chronically transfused patient populations reported, but higher than that reported in transfused patients at large. The incidence of positive DATs was higher than in some populations reported but not others.