STRING THRESHOLD CORRECTIONS AND FLIPPED SU(5)

We revise previous calculations of the effective unification scale m(SU) at which the extrapolated low-energy gauge couplings should appear to become equal, and we show explicitly how to calculate m(SU) in the fermionic construction of four-dimensional strings. In the case of the flipped SU(5) GUT derived from the string, the SU(5) and U(1) couplings defined in the DRBAR scheme become equal to g(SU) at m(SU) congruent to 1.76 X g(SU) X 10(18) GeV. This scale is significantly larger than m(GUT), the scale at which the low-energy SU(3) and SU(2) couplings become equal if extrapolated using the renormalization group equations of the minimal supersymmetric extension of the standard model. The existence of an intermediate SU(5) X U(1) phase could have an observable effect on the calculated value of sin2-theta-w.