After receiving apparently compatible blood three patients suffered hemolytic reactions. The compatibility tests were by saline and indirect Coombs technique including a screening tube of group 0 cells. The antibodies responsible for these reactions were not clearly demonstrable for several days following the transfusion. In two instances the antibody was anti-E.These case reports point up the following. (a) Currently used cross-matching procedures will occasionally fail to demonstrate an incompatibility. (b) In two of the cases the direct Coombs test was negative on an immediate post-transfusion specimen, when it could have been of great aid in diagnosis. (c) When a transfusion reaction of hemolytic type is suspected, a follow-up study several days after the reaction may clarify the diagnosis; when possible, transfusions should be avoided in the interim.