We have compared the binding properties of c-JUN, JUN B, and JUN D in the absence or in the presence of c-FOS, FOS B, and FRA-1 to different AP-1 and CRE-containing oligonucleotides. The results demonstrate that for a given AP-l-containing oligonucleotide the binding affinities of the different JUN proteins are always c-JUN> JUN D> JUN B. The three JUN proteins have the capacity to bind to a CRE consensus sequence with very high affinity. We have found that c-JUN, JUN B, and JUN D bind with different affinities to different oligonucleotides containing an identical AP-1 or CRE binding site, implying that the adjacent sequences influ¬ ence the stability of the JUN/DNA complexes, inter¬ estingly, an AP-l-containing oligonucleotide which binds the JUN proteins with high affinity can be converted to a CRE-containing oligonucleotide which will also bind the different JUNs very efficiently. The heterodimers formed between the different JUN and FOS proteins have an enhanced binding activity compared to the JUN: JUN homodimers. In all cases the half-lives of the JUN: FOS/DNA complexes are longer than those of the JUN: JUN/DNA complexes. The most stable complexes were obtained in the presence of FOS B, followed by FRA-1 and c-FOS.