Proteins encoded by early region 1A (E1A) regulate transcription of viral and cellular genes. The mechanism of this trans-activation is not understood, but is of considerable interest as an example of transcription regulation through a cellular intermediate. We have therefore studied the effect of E1A products on the activity of the kappa L chain gene promoter and enhancer. By using transient and stable transfections into lymphoid and nonlymphoid cells, we found that the E1A proteins have a pleiotropic effect on the regulation of the mouse kappa-chain gene enhancer. In lymphoid cells the E1A products repress kappa-chain enhancer, whereas in fibroblasts, the kappa-chain enhancer is activated by the E1A products whether the E1A gene is in an extrachromosomal location or stably integrated in the genome. Furthermore, a functional kappa-chain promoter, containing the octanucleotide and "TATA" sequences is needed in order to be transcribed in E1A-producing cells. This ability of E1A products to negatively and positively regulate kappa-chain transcription may reflect a more general phenomenon in which a given cellular protein could participate in a variety of different cellular controls.