The inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase by citrinin, an antibiotic, has been studied. The inhibition was the mixed type with respect to 3-hydroxy-3-methylglutaryl-CoA and non-competitive with respect to NADPH. When the enzyme was preincubated with citrinin prior to enzyme assay, however, it caused a time-dependent, irreversible inhibition, possibly by binding to a site distinct from the active center on the enzyme protein.