Proteins specified by the mini-F plasmid (EcoRI restriction fragment f5) were labeled in Escherichia coli minicells and analyzed by SDS-PAGE. Four mini-F encoded proteins could be identified, having molecular weights of 44,000 (A), 36,000 (B), 34,000 (C), and 25,300 (D) daltons. The absence of certain proteins in deleted derivatives of mini-F, generated by treatment with various restriction endonucleases, allowed mapping of the proteins. The A protein maps between F-coordinates 45.7 and 47.9 kb. The gene locus for the B protein is located between 47.2 and 49.3 kb. The C protein maps on a BamHI fragment bordered by F-coordinates 41.5 and 42.8 kb, and finally the D protein maps between 42.8 and 43.8 kb. In addition our data confirm that there are two incompatibility loci on the mini-F genome, located between 45.7 and 47.2 kb (incA) and 44.0 and 45.7 kb (incB). We suggest that (i) the C and D proteins are positive control elements, interacting with origin I and origin II, respectively, (ii) that the incB locus is involved in plasmid partitioning, and (iii) that the A protein encoded by the incA locus is a negative control element.