The cell-cycle replication patterns of two mini-F plasmids have been examined using the membrane-elution technique (to produce cells labelled at different times during the division cycle) and scintillation counting (for quantitative analysis of radioactivity incorporated into plasmid DNA). The mini-F plasmid pML31, which contains the oriV and oriS origins of replication, replicates in a cell-cycle-specific manner with a pattern and cell-cycle timing similar to the parental F plasmid. The mini-F plasmid pMF21, deleted for the region containing the oriV origin of replication, replicates more randomly throughout the division cycle. These results suggest that the oriV origin of replication may be related to cell-cycle-specific replication of the F plasmid.