Studies of the survival time of 51Cr labeled erythrocytes treated in vitro with methyl acetimidate (MAI) were conducted in 13 patients with sickle cell disease in order to assess the suitability of this antisickling agent for more extensive clinical testing. In comparison with previously measured control values (average t1/2 8.4 +/- 1.1 days a), the survival time of the treated erythrocytes in 10 of the patients who were not transfused was initially prolonged (average t1/2 24.4 +/- 4.6 days). However, 5 of the 13 patients studied developed circulating antibody against the MAI treated erythrocytes, markedly reducing the survival time of MAI treated erythrocytes in subsequent studies. Two patients, each challenged 3 times with infused MAI treated erythrocytes, failed to show evidence of antibody production, suggesting that not all subjects become immunized even after repeated exposure. In spite of many other promising properties of MAI as an antisickling agent of potential value, consideration of its use in further clinical testing must depend on successful avoidance of immunization in patients receiving infusions of treated erythrocytes.