Improved methods for production, isolation, and assay of two new chlorisoxazoline amino acid antitumor antimetabolites: U-42, 126 and U43, 795.

Improved fermentation and isolation procedures for antitumor antimetabolites U-42,126 and U-43,795 increased drug yields 30-fold. The sensitivity limit of a newly developed assay is 0.03 mug of U-42,126 and 2.0 mug of U-43,795 per ml. The in vitro antimicrobial effect of both drugs was antagonized by histidine.