Transcription of soluble methane monooxygenase (sMMO) of methanotrophs is tightly regulated by low concentrations of copper ions [Cu(II); e.g., transcription is completely repressed al copper concentrations higher than 0.86 mu mol/g dry cell weight]. In addition to this genetic-level regulation, copper ions have been shown to inhibit the in vitro activity of sMMO from the type X methanotroph Methylococcus capsulatus (Bath) by inactivating only the reductase component of this enzyme (Green et al. 1985). In this study, in vitro sMMO inhibition by 12 metal ions and 10 medium ingredients was investigated for the first time using sMMO purified from the type II methanotroph Methylosinus trichosporium OB3b. Cu(I) and Cu(II) decreased sMMO activity of Methylosinus trichosporium OB3b by inhibiting not only the reductase but the hydroxylase component as well. Ni(II) also inhibited both enzyme components, but the inhibition was weaker than with copper ions. Zn(II) inhibited sMMO by lowering the activity of the hydroxylase only. Other transition metals such as Co(II), Mn(II), Fe(II) and Fe(III) did not show considerable impact on sMMO activity. The inhibition mechanisms were not determined, but Ni(II) and Zn(II) aggregated the reductase component of sMMO, and Zn(II) also precipitated the hydroxylase component, Cu(II) caused the reductase to precipitate, but Cu(I) did not aggregate either sMMO component. The aggregated proteins could not be dissolved in the solution of ethylenediaminetetraacetic acid disodium salt, Little or no sMMO inhibition was observed with various medium components examined including glucose, methanol, ethanol, dimethyl sulfone, ammonium chloride, methylamine (at a 500 molar ratio of medium component to the hydroxylase) : kanamycin, and isopropylthiogalactopyranoside (at a molar ratio of 50); however, chloramphenicol inhibited sMMO at a molar ratio of 50.