Drug resistance has been shown to be associated with the expression of P-glycoprotein (P-gp), the product of mdr-1 gene. One of the suggested mechanisms for the phenomenon called Multidrug Resistance (MDR) is related to the overexpression and amplification of the mdr-1 gene. The product of this gene is called P-gp and it has been considered as a potential marker for drug resistance. Transfection of the mdr-1 gene into drug-sensitive cells confers the role of mdr-1 gene in developing MDR phenotype. Structural analysis of homologous cDNA, responsible for production of transport proteins, from: MDR cell lines, bacteria and yeast, revealed high similarity. Molecular structure analysis indicate that P-gp has nucleotide binding sites. It has been established that P-gh has an internal ATP-ase activity, thus it can act as energy dependent transport protein. The expression of P-gp has been found in neoplastic and normal tissues (as adrenal glands, kidney, liver, pancreas, jejunum and large intestine), as well as in several cell lines which have been induced to become resistant to cytostatics. The aim of present study was to review the role of P-gp expression in laboratory and clinic.