Stirling & Dalton (1978) isolated a formaldehyde dehydrogenase (mFDH) from Methylococcus capsulatus (Bath) which required a small heat stable component for the catalysis of formaldehyde oxidation to occur. The mFDH was purified and shown to be a dimer of 57kDa subunits. The heat stable component was purified and shown to modify the function of FDH and was therefore termed a modifier protein (protein F) (Millet et. al., unpublished). In this study purification procedures for protein F and mFDH are described. The N-terminal sequence of purified mFDH and protein Fare described and data are presented which indicates that mFDH is a tetramer of 63 kDa subunits. It was known that protein F altered the function of mFDH from a specific FDH to a general aldehyde dehydrogenase. The data presented in this study demonstrate that only formaldehyde oxidation is catalysed in the presence of protein F and in its absence fonnaldehyde oxidation can not be detected. The data also show that the oxidation of a range of aldehydes and alcohols are catalysed by mFDH in the absence of protein F. The data demonstrate that formaldehyde association to mFDH is cooperative resulting in a sigmoidal plot for rate vs. formaldehyde concentration while acetaldehyde oxidation follows Michaelis-Menten kinetics. Data are also presented which demonstrate that protein F may induce a conformational change in mFDH which allows formaldehyde oxidation catalysis to occur and inhibits the oxidation of acetaldehyde. The data also show that protein F has limited effects on other dehydrogenase enzymes. An enzyme whose activity is altered by protein F was identified in a commercial sample of calf liver glucose dehydrogenase (GDH) and this was purified and shown not to be GDH. Also described is the characterisation of a second NAD+linked FDH (nFDH) isolated from Methylococcus capsulatus (Bath) which does not require a modifier protein. From the data presented it is proposed that the function of mFDH may be to aid in reducing toxic formaldehyde concentrations in the organism and the role of the nFDH could be the generation of NADH for methane and carbon assimilation.