A process for making pure crystalline D-isoglutamyl-D-tryptophan is provided which includes the step of deprotecting essentially pure N-tert-butoxycarbonyl-D-isoglutamyl-D-tryptophan or its diester to yield essentially pure D-isoglutamyl-D-tryptophan. A process is also provided for the preparation of pure mono ammonium salt of D-isoglutamyl-D-tryptophan from essentially pure N-tert-butoxycarbonyl-D-isoglutamyl-D-tryptophan. D-isoglutamyl-D-tryptophan, ammonium salt (1:1) is a stable pharmaceutical solid.