Varying capacities for autonomous replication have been obtained with bovine papillomavirus type 1 (BPV-1)-based expression vectors in mouse C127 cells. Both integration of the vector DNA into the genome of the host cell and replication as monomeric extrachromosomal elements have been observed. In this study, we have examined what features of BPV-1 vectors influence their replication potential. Transfection of the entire BPV-1 genome into C127 cells resulted in the replication of extrachromosomal monomeric BPV-1 elements. The same result was obtained when a plasmid sequence was inserted into the BPV-1 DNA. However, introduction of foreign, transcriptionally active units resulted in chromosomal integration of the expression vectors. This result was obtained with clones isolated by co-transfection followed by neomycin selection, as well as with clones isolated from neoplastic foci. Supertransfection of a BPV-1-based expression vector into cells harbouring unintegrated replicating BPV-1 genomes resulted in integration of the vector DNA, whereas replication of the resident BPV-1 genomes was unaffected. Extrachromosomal replication of such a vector was achieved when the enhancer and promoter region of the foreign gene were deleted.