When membrane bound E. coli or B. subtilis DNA is digested with EcoR1, the remaining fast sedimenting material is enriched for branched fragments. We have shown that these branches display many of the properties expected of replicating growing points. Our present goal is to characterize these fragments in more detail and to use the technique to isolate replicative intermediate fragments from higher organisms. We are interested in obtaining new information on directionality of replication. Why have some genomes evolved a bidirectional mechanism while others exhibit unidirectionality? A model is being tested which rests on the simple notion that initiation of replication will naturally lead to bidirectionality unless a specific event takes place to prevent propagation of one of the growing forks. This idea is being tested by (1) attempting to change directionality of P2 replication, (2) examination of a recently discovered bias in directionality of lambda replication, and (3) examination of the transition from early to late replication in lambda. Electron microscopy of DNA, and in particular electron microscopic partial denaturation mapping, required many time-consuming and error-prone length measurements. We are testing to see if there is, at present, any merit in trying to make length measurements by automatic means.