Human folate binding proteins (FBP), or folate receptor (FR), bind and transport physiologic reduced folates and methotrexate, a chemotherapeutically-active folate analogue. Others have reported that defects in drug influx account for acquired methotrexate drug resistance in 40-60% of mutant cell lines studied. The Experimental Hematology Section is investigating the following: 1) Structure, Function, and Molecular Biology of Human FRs; 2) Role of the FRs in folate/drug transport and acquired resistance; 3) Expression of the FRs.