The condensation of O4-deacetyl-vinblastine carboxazide (I) with diethyl ester (+) of 1-amino-2-methylpropylphosphonic acid (II), in cold methylene chloride, gives (1S)-1-[3-(O4-deacetyl-3-demethoxycarbonylvincaleukoblastinyl) carbonylamino]-2-methylpropylphosphonic acid diethyl ester (III).