The presence of a multidrug resistance (MDR) related protein, P-170, in normal and pathological lymphoid cells has been described. The present report evaluates the expression of the mdr 1 gene by using the reverse Polymerase Chain Reaction (PCR) on cells obtained from the thymus and bursa of chicken embryos starting from day 12 until hatching. Results show that the thymic cells are positive from day 12 to the end of the observation period. In contrast, mdr mRNA was detected in the bursa from day 14 to day 17 of embryonic life. Possible relationships between the expression of mdr and the development of T and B lymphocytes are discussed.