Most drug-induced immune hemolytic anemias since the late 1980s have been caused by the second- and third-generation cephalosporins, cefotetan and ceftriaxone, respectively. Cross-reactivity of cefotetan and ceftriaxone antibodies with other cephalosporins or penicillin has been studied only minimally. We tested 7 serum samples previously identified to contain cefotetan antibodies and one serum sample previously identified to contain ceftriaxone antibodies against 9 other cephalosporins, penicillin, and 7-aminocephalosporanic acid in the presence of RBCs and also used hapten inhibition to indicate cross-reactivity. Serum samples containing cefotetan antibodies showed some cross-reactivity with cephalothin and cefoxitin (and to a much lesser extent with penicillin and ceftazidime). The ceftriaxone antibodies showed very weak cross-reactivity with cefotaxime, cefamandole, and cefoperazone. There was very little cross-reactivity between cefotetan antibodies and the drugs tested in the present study. We have no data to determine whether the in vitro data relate to in vivo reactivity.