Cross-reaction rate constants k(12) (22 degrees C) at pH 7.0 have been determined for the reduction of Fe-2(III) and tyrosyl-radical-containing active-R2 from E. coil ribonucleotide reductase with eight organic radicals (OR), e.g., MV.+ from methyl viologen. The more reactive OR's were generated in situ using pulse radiolysis (PR) techniques, and other OR's were generated by prior reduction of the parent with dithionite, followed by stopped-flow (SF) studies. In both procedures it was necessary to include consideration of doubly-reduced parent forms. Values of k(12) are in the range 10(9) to 10(4) M-1 s(-1) and reduction potentials E-1(o) for the OR vary from -0.446 to +0.194 V. Samples of E. coil active-R2 also have an Fe-2(III) met-R2 component (with no Tyr(.)), which in the present work was close to 40%. From separate experiments met-R2 gave similar kit rate constants ton average 66% bigger) to those for active-R2, suggesting that reduction of the Fe-2(III) center is the common rate-limiting seep. A single Marcus free-energy plot of log k(12) - 0.5 log f vs -E-1(o)/0.059 describes all the data, and the slope of 0.54 is in satisfactory agreement with the theoretical value of 0.50. It is concluded that the rate-limiting step involves electron transfer. In addition, the intercept at -E-1(o)/0.059 = 0 is 5.94, where values of the reduction potential and self-exchange rate constant for met-R2 contribute to this value. To maintain electroneutrality at the similar to 10 Angstrom buried active site H+ uptake is also required. For both e(-) and H+ transfer the conserved pathway Trp-48, Asp-237, His-118 to Fe-A is a possible candidate requiring further examination.