Human cell lines resistant to L-asparaginase or albizziin were isolated by multistep selection of HT1080 fibrosarcoma and MIA PaCa-2 pancreatic carcinoma cells. Mutants were cross-resistant to both drugs, but more resistant to the drug used for selection. The drug-resistant cell lines expressed elevated levels of asparagine synthetase activity and protein, up to 17-fold over that of the parental cells. Enzyme overproduction was due to gene amplification in the albizziin-resistant cells, whereas increased expression without amplification was observed in L-asparaginase-resistant cells.