Transversion mutations can be distinguished from transition mutations by the use of special tauII mutants of bacteriophage T4. Methyl methanesulfonate did not induce reversion of the tester mutants along transversion or transition pathways from A:T1 base pair sites, nor along transversion pathways from G:C base pair sites. Ethyl methanesulfonate and N-methyl-N-nitrosourea, however, induced both transversions and transitions at an A:T base pair site; no transversions were detected at G:C-sites. Mn++ induced transversions and transitions at both A:T-and G:C-sites. The influence of temperature-sensitive gene-43 DNA polymerase mutator and antimutator mutations on the reversion of the tauII tester mutants was measured: some gene-43 mutants differentially influenced different pathways of reversion. Studies of thymineless mutagenesis demonstrated A:T-site transversion mutations. A synergistic interaction between thymineless mutagenesis and the gene-43 mutator, tsL56, was used to demonstrate thymineless mutagenesis at one site where it was not detected in the presence of the wild type polymerase.