This study compares the rates of lambda protein synthesis after infection of rnc- cells, which are defective in ribonuclease III (RNase III), with the analogous rates in an isogenic rnc+ host. Temporal differences in gene expression are reflected in a delay in turn-off of lambda early proteins as well as in the delayed appearance of late phage functions in rnc- host cells. Moreover, in the two hosts there is a striking difference in the regulation of gene int expression, which in wild-type cells requires the product of the lambda cII (and cIII )genes, whereas Int synthesis occurs in the absence of cII in RNase III-defective cells. These results suggest that RNase III may be a negative regulator of Int synthesis. The expression of int is also shown to be cII- and cIII-independent in rnc+ cells infected with b2-deleted phages, thus confirming previous studies on the negative regulation of int by the b2-region. Possible mechanisms of these two inhibitory effects on int expression are considered and the significance of int regulation in the control of site-specific recombination is discussed.