Host functions for DNA replication of bacteriophage alpha3, a representative of group A microvirid phages, were studied using dna and rep mutants of Escherichia coli. In dna+ cells, conversion of phage alpha3 single-stranded DNA (SS) into the double-stranded replicative form (RF) was insensitive to 30--150 microgram/ml of chloramphenicol, 200 microgram/ml of rifampicin, 50 microgram/ml of nalidixic acid, or 200 microgram/ml of novobiocin. At 43 degrees C, synthesis of the parental RF was inhibited in dnaG and dnaZ mutants, but not in dnaE and rep strains. Replication of phage alpha3 progeny RF was prevented by 50 microgram/ml of mitomycin C (in hcr+ bacteria), 50 microgram/ml of nalidixic acid or 200 microgram/ml of novoviocin, but neither by 30 microgram/ml of chloramphenicol nor by 200 microgram/ml of rifampicin. Besides dnaG and dnaZ gene products, dnaE and rep functions were essential for progeny RF synthesis. Host factor dependence of alpha3 was relatively simple and, in contrast with phages phiX174 and G4, alpha3 did not require dnaB and dnaC(D) activities.