Sirohaem is a new type of haem that has been detected as a prosthetic group of several bacterial and plant enzymes that catalyse the six-electron reductions of sulphite to sulphide or of nitrite to NH(3). When a methionine-requiring mutant of Escherichia coli K12 was grown on a minimal medium supplemented with d-glucose and l-[Me-(3)H]methionine, 2.4 methyl groups per spectrophotometrically detectable haem group were incorporated into the sirohaem prosthetic group of the NADPH-sulphite reductase isolated from the organism. When the same strain of cells was grown on minimal medium supplemented with d-[U-(14)C]glucose and l-[Me-(3)H]methionine, the sirohaem isolated was found to contain a ratio of glucose-derived carbon/methionine-derived methyl of 19.8. This ratio is in excellent agreement with the value of 20 predicted by the iron-dimethyl-urotetrahydroporphyrin structure for sirohaem proposed by Murphy, Siegel, Kamin & Rosenthal [(1973) J. Biol. Chem.248, 2801-2814]. It can be concluded that sirohaem is indeed methylated, with the methyl groups derived from methionine (rather than by modification of existing side chains, as in protohaem). The structure proposed by Murphy et al. (1973) is therefore probably correct in its essential features. A possible relationship between the pathway for biosynthesis of sirohaem and that for synthesis of vitamin B(12) is discussed.