A stable transition state analogue for an aspartate transcarbamylase- cartalyzed reaction with antineoplastic activity. N-phosphonacetyl-L-aspartate (PALA) is a stable transition analogue of the activated complex for the reaction catalyzed by aspartate transcarbamylase, the first step in the pyrimidine biosynthetic pathway. This agent inhibits de novo pyrimidine biosynthesis and increases the extent to which fluorouracil metabolites are incorporated into RNA.