Studies of electron transfer by biological oxidation-reduction proteins frequently focus on the interaction of a particular protein with nonphysiological oxidants and/or reductants. This approach, although valuable, is limited by the size and chemistry of the nonphysiological reactants. To further the understanding of biological electron transfer, we have investigated the interaction of two examples of high-potential iron-sulfur proteins (HIPIP's) with mitochondrial cytochrome c (horse heart) and bacterial cytochrome c2 from Rhodospirillum rubrum, Rhodopseudomonas palustris, Rhodopseudomonas capsulata, and Rhodopseudomonas sphaeroides. On the basis of the kinetics of electron transfer between the various HIPIP's and cytochromes, it appears that the interactions are more complex than those observed with nonphysiological reactants. We conclude that (1) specific sites on both the HIPIP's and the cytochromes mediate electron transfer with the effect of ionic strength different from that expected on the basis of the interaction of the various proteins with iron hexacyanides, (2) the interaction of HIPIP with some of the cytochromes investigated is heterogeneous, resulting from at least two possible orientations (cytochrome dependent) for interaction leading to electron transfer, and (3) no long-lived complexes between the HIPIP's and cytochromes are formed due to rapid equilibrium between the two proteins. This last conclusion suggests that the measured second-order rate constant is in fact the product of the association constant (for any HIPIP and a particular cytochrome) and a first-order rate constant reflecting the rate-limiting step leading to products.