Resistance of Enterobacter, Klebsiella and Pseudomonas aeruginosa to cefotaxime, cefoperazone, mezlocillin, azlocillin and piperacillin was induced by 15 culture transfers at subinhibitory concentrations. Information about a possible cross resistance between cephalosporins and penicillins was obtained by determining the MIC's of the five beta-lactam antibiotics studied before and after resistance was induced. The highest homologous resistance is induced in Enterobacter and Ps. aeruginosa by cefotaxime (factors 2021 and 42.2, respectively), and in Klebsiella by cefoperazone (factor 64). The lowest increase in homologous resistance is induced in Ps. aeruginosa by cefoperazone (factor 9.9), and in Enterobacter and Klebsiella by mezlocillin (factor 10.6 and 4.6 respectively). Resistance induced by cephalosporins is often accompanied by resistance to penicillins. Such cross resistance is observed especially in Ps. aeruginosa and Enterobacter. Resistance to cephalosporins is, however, only rarely induced by penicillins. These findings permit the conclusion that the use of all beta-lactam antibiotics may be prohibited by uncritical treatment with cephalosporins. Preference should therefore be given to acylureidopenicillins in primary treatment of emergency cases.