One aspect of the present invention is a purified human GMP synthetase and a method of purifying it from naturally occurring sources. Another aspect of the present invention is a recombinant human GMP synthetase as well as DNA sequences coding for human GMP synthetase, expression vectors comprising such a coding sequence, and host cells transformed with these expression vectors capable of producing human GMP synthetase. Also forming part of this invention is a recombinant process for the production of GMP synthetase. Further provided is a method of purifying GMP synthetase from natural or recombinant sources. Other aspects of the invention include antibodies to human GMP synthetase and the use of such antibodies to assay for human GMP synthetase. Another aspect of this invention is the use of purified naturally occurring human GMP synthetase or recombinant human GMP synthetase to identify inhibitors of GMP synthetase activity. Another aspect of the invention is inhibitors of human GMP synthetase activity obtained using purified human GMP synthetase. Such inhibitors preferably have an IC.sub.50 of 5 .mu.M or less, more preferably 1 .mu.M or less.