Pulse electron-electron double resonance (PELDOR) has been employed to measure the distance between the putative tyrosyl radicals in the two halves of the R2 subunit from mouse ribonucleotide reductase. The results provide experimental evidence that the active, tyrosyl radical containing mouse R2 subunit forms a homodimeric form in solution. The distance between the two tyrosyl radicals present in the dimer was determined to be 3.25 +/- 0.05 nm.