A positive cis-acting element, the B element, located between -83 and -61 in the mouse alpha 1(III) collagen promoter, binds a factor present in nuclear extracts of NIH 3T3 fibroblasts and HeLa cells. We have purified this factor using ion exchange chromatography, sequence-specific DNA affinity chromatography, and sodium dodecyl sulfate-polyacrylamide gel fractionation. The DNA sequence used for the affinity chromatography was a single-base substitution in the B element that increased the stability of the B element-protein complex by 50%. Purification of the B element-binding factor (BBF) by DNA affinity chromatography resulted in the apparent loss of most or all of the DNA-binding activity of this factor. The DNA-binding activity could, however, be reconstituted by combining two chromatographic fractions: the high-salt eluate and the column flow-through. When the partially purified high-salt eluate was size-fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with subsequent renaturation of gel fractions from guanidine HCl, the purified BBF (apparent molecular weight of about 95,000) bound to the B element with high affinity. These results suggest that during DNA affinity purification of BBF a factor that inhibits BBF DNA binding was co-eluted with BBF. This inhibition of BBF DNA binding was reversed by the addition of the DNA affinity column flow-through. The binding of BBF to the B element of the mouse alpha 1(III) collagen promoter is therefore an apparently complex process involving interactions between BBF and other protein factors.