The survival of red blood cells (RBCs) radiolabeled with 51Chromium (51Cr) is a reliable method for predicting transfusion compatibility. Approximately 1.0 ml of 51Cr tagged RBCs is infused into the patient and samples are drawn at predetermined intervals post infusion to determine RBC survival. Red cells used for the study are usually incompatible with the patient's antibody. This antigenic rechallenge may stimulate further antibody production, which could contribute to accelerated destruction of RBCs during subsequent incompatible transfusions. To study this question, blood samples were collected from six patients with a single atypical alloantibody, seven to nine months after 51Cr RBC survival studies had been performed in an unrelated protocol. Blood had not been transfused in the interim. The samples were tested in parallel with pre-survival samples for change in antibody titer and score. In addition, RBCs used for the 51Cr survival study, sensitized in vitro with pre- and post-survival serum, were tested by the monocyte monolayer assay (MMA). Serum antibody titers showed a significant increase (> 2 dilutions) in two of six patients and antibody score was significantly increased (> 10) in three of six patients. One of the two negative MMAs became weakly positive. This study suggests that 51Cr survival tests using small volumes of RBCs can stimulate significant increases in antibody in some patients, a factor that must be considered in performing these tests and in planning subsequent transfusions.