Background : The MDR1 (multidrug resistance) gene (also known as PGY1), which encodes the transmembrane efflux pump P-glycoprotein (Pgp), confers resistance to Pgp-transported cytotoxic drugs in tissue culture. The increase in MDR1 expression in tumors after chemotherapy is usually attributed to selection of pre-existing multidrug-resistant cells by Pgp-transported drugs. MDR1 expression in tissue culture can be increased by several types of stress-inducing treatment, including agents that activate protein kinase C (PKC). Previous studies, however, failed to demonstrate that short-term exposure to any chemotherapeutic drug can induce the expression of the resident MDR1 gene in drug-sensitive human cells. Purpose : This study was designed to utilize highly sensitive assays to determine if transient exposure to chemotherapeutic drugs would have an effect on MDR1 expression in human cells and to assess if PKC inhibitors would influence such an effect Methods : We analyzed the MDR1 gene expression in several human cell lines derived from leukemias or solid tumors, after treatment with different cytotoxic drugs, in the presence or absence of PKC inhibitors. Pgp function and expression were studied by flow cytometric assays, and MDR1 messenger RNA (mRNA) was assayed by polymerase chain reaction. Results : Transient exposure to chemotherapeutic drugs, including agents that are not transported by Pgp, induced Pgp and MDR1 mRNA expression in subpopulations of treated cells in most of the tested cell lines. This induction was observed along with microscopically detectable cell damage. The drug-induced MDR1 expression and the associated twofold to threefold increase in resistance to vinblastine were sustained in K562 leukemia cells for at least several weeks after the removal of the drug. Drug-mediated MDR1 induction was blocked by nonspecific protein kinase inhibitors that are active against PKC, but not by a protein kinase inhibitor ineffective against PKC. Conclusions : Expression of the human MDR1 mRNA and Pgp can be induced in response to cellular damage by cytotoxic drugs regardless of whether the drugs are transported by Pgp. This induction can be prevented by protein kinase inhibitors. Implications : Induction of MDR1 expression in response to cellular damage may account for increased MDR1 expression in treated human tumors. Protein kinase inhibitors may be useful in preventing the emergence of multidrug resistance during cancer chemotherapy. [J Natl Cancer Inst 85:632-639, 1993]