We have studied the plasmid x plasmid recombination system to examine factors that might increase the frequency of homologous recombination. Treatment of input DNA with UV does up to 100 Jm-2 did not have a significant effect on recombination, on the other hand , treating the recipient EJ cells with UV light at a dose of 1Jm-2 prior to transfection resulted in a two fold increase in recombination. The tumor promoter TPA treatment did not have an effect on recombination or transformation. We have studied the effect of heterologous inserts on homologous recombination and observed that as the length of insert increased, there was decline in the frequency of recombination.