Calcium-treated cells of E. coli K-12 C600 were transfected with lambda-heteroduplex DNA carrying the marker cIts857 in one strand and wildtype in the other. In single burst analyses of the phage progeny, 72-79% of the bursts were "pure" bursts containing either exclusively wildtype phage or exclusively mutant phage, indicating that conversion of the cIts857/+ mismatch to a homoduplex structure prior to replication occurred with this frequency. The r-strand1 appears to be "preferred", since pure bursts of progeny with the r-strand genotype were almost twice as frequent as those with the l-strand genotype. Examination of the conversion frequency of a number of rec and uvr E. coli mutants showed that the mutants uvr D and UVR E are deficient in mismatch repair. Conversion is reduced in the former by a factor of 2 and in the latter by a factor of 3.