We have previously suggested that a repeated sequence motif in the upstream region of the human cardiac actin gene 'CC.Ar.GG', where Ar is an (A + T)-rich six-base-pair-sequence, may be important in the muscle-specific expression of this gene [Minty, A. & Kedes, L. (1986) Mol. Cell Biol. 6, 2125-2136]. Here we show that this sequence binds a nuclear protein, and that binding is abolished by mutating either the CC and GG dinucleotides or the (A + T)-rich centre. Mutation of the CC and GG nucleotides also abolishes the transcription-stimulating activity of this sequence on the cardiac actin promoter. A similar sequence has been implicated in the serum-response of the c-fos gene [Treisman, R. (1986) Cell 46, 567-574]. We show that this c-fos 'CC.Ar.GG' sequence competes with the cardiac actin sequence for factor binding. Our results suggest that the minimum sequence requirements for binding of the serum response factor may correspond to the 'CC.Ar.GG' box sequence. Using this criterion, we predict and confirm the existence of such a binding site in a regulatory region of the interleukin-2 receptor gene. It appears therefore that interactions between 'CC.Ar.GG' boxes and similar proteic factors could be involved in the control of different genes responding to different stimuli, e.g. muscle differentiation (cardiac actin gene) or growth stimulation (c-fos, cytoskeletal actin or interleukin-2 receptor genes).