Folic acid binding protein (FABP) has been measured in the supernatant of leukocytes of 12 patients affected with chronic granulocytic leukaemia (CGL). The folate binding capacity ranged from 1.37 to 697.52 pg/mg protein, showing a considerable heterogeneity. When the supernatant was preincubated with methotrexate (MTX), the inhibition of folic acid binding was complete in some cases whereas in others it was negligible: these findings have been confirmed by studying the 3H-MTX binding capacity by the same supernatants. In this case the range of bound 3H-MTX varied from 0.00 to 927 pg/mg protein. The presence of a binder in the cytoplasm of leukocytes might represent a new step in the regulation of endogenous folate metabolism. The MTX, widely used as an antifolate drug, may also be bound by FABP of CGL leukocytes, although in different amounts from case to case: this finding suggests a new point of interference of MTX in the folate metabolism. It has also been demonstrated that FABP, which is present in serum, may reduce the uptake of folate by leukocytes opening a new field of investigation on the megaloblastic transformation.