ALTHOUGH a number of methods for estimating aliphatic nitro compounds have been described, all have various shortcomings. They are either insensitive1, non-specific2, require special equipment3,4, or are rather complicated to carry out5,6. No one seems to have based an analytical procedure on the observation that primary aliphatic nitro compounds when hydrolysed by sulphuric acid yield hydroxylamine7. This is a very specific reaction. To my knowledge, the only other compounds which yield hydroxylamine on acid hydrolysis are oximes. Hydroxylamine can be determined readily by oxidizing it with iodine to form nitrous acid8, which in turn is determined colorimetrically by diazotizing sulphanilic acid and coupling with α-naphthylamine9. I developed a procedure based on these reactions for analysing culture filtrates of mutants of Streptomyces venezuelae in an investigation of the biosynthesis of the antibiotic, chloramphenicol10. The procedure has proved to be specific, sensitive and easy to perform.