The DinI and RecX proteins of Escherichia coli both modulate the function of RecA protein, but have very different effects. DinI protein stabilizes RecA filaments, preventing disassembly but permitting assembly. RecX protein blocks RecA filament extension, which can lead to net filament disassembly. We demonstrate that both proteins can interact with the RecA filament, and propose that each can replace the other. The DinI/RecX displacement reactions are slow, requiring multiple minutes even when a large excess of the challenging protein is present. The effects of RecX protein on RecA filaments are manifest at lower modulator concentrations than the effects of DinI protein. Together, the DinI and RecX proteins constitute a new regulatory network. The two proteins compete directly as mainly positive (DinI) and negative (RecX) modulators of RecA function.