ABSTRACT We developed a system to monitor the transfer of heterologous DNA from a genetically manipulated strain of Saccharomyces cerevisiae to Escherichia coli . This system is based on a yeast strain that carries multiple integrated copies of a pUC-derived plasmid. The bacterial sequences are maintained in the yeast genome by selectable markers for lactose utilization. Lysates of the yeast strain were used to transform E. coli . Transfer of DNA was measured by determining the number of ampicillin-resistant E. coli clones. Our results show that transmission of the Amp r gene to E. coli by genetic transformation, caused by DNA released from the yeast, occurs at a very low frequency (about 50 transformants per μg of DNA) under optimal conditions (a highly competent host strain and a highly efficient transformation procedure). These results suggest that under natural conditions, spontaneous transmission of chromosomal genes from genetically modified organisms is likely to be rare.