The effect of multiplicity of infection was studied in Escherichia coli with lambda phage, using phage endolysin as an example of a late gene product. A very sensitive endolysin assay method was used so that the initiation time of endolysin synthesis could be more accurately determined. It was observed that high multiplicity of infection (1) increases the rate of lysogenization, (2) progressively delays lysis time, and (3) significantly delays and reduces the synthesis of endolysin in lamdacIII+ cII+ -infected cells. The extent of delay and reduction in endolysin synthesis increases with increasing multiplicity. In contrast, lamdacIII67cII68-infected cells show no delay in endolysin synthesis at high multiplicity of infection when compared with the lamdacIII+ cII+ -infected cells. The results suggest that (1) the expression of cIII and cII genes is multiplicity dependent, (2) high multiplicity of infection enhances the expression of the cIII and cII genes, and (3) the expression of the cIII and cII genes interferes with the expression of the late genes. A model to explain how the expression of the cIII and cII genes interferes with the expression of the late genes is proposed.