FixL is a ferrous heme protein whose kinase activity is inhibited by oxygen. Here we show that met-FixL, which is the ferric unliganded form, has the same activity as deoxy-FixL, the ferrous unliganded form, indicating that activity does not depend on the oxidation state of the heme iron. The ferric derivative fluoro-FixL is fully active, indicating that the presence of a heme ligand is not sufficient to cause kinase inhibition. An inverse relation between the rate of autophosphorylation of ferric FixL and the fractional saturation with cyanide shows that the cyanomet form has zero activity. All our active derivatives were high-spin, while our inactive derivatives were low-spin. In mixtures of high- and low-spin FixL, resulting from partial saturation with low-spin ligands, the activity was that which would be expected for the concentration of the high-spin component alone. Therefore the spin state of the heme iron rather than the oxidation state or presence of ligands must be the factor that controls FixL's kinase activity. On transition from low to high spin, the heme iron moves out of the porphyrin plane by 0.4 A. We propose that, as in hemoglobin, this motion triggers a long-range conformational change which in FixL is responsible for a switch to an active form.