Bacteriophage T4 ribonucleoside diphosphate reductase consists of alpha 2 and beta 2 subunits encoded by genes nrdA and nrdB, respectively, and plays a central role in the T4-induced deoxyribonucleotide synthetase complex. The accompanying paper describes the decreased rate of synthesis of deoxyribonucleotides after infection by the T4 mutant, nrdB93, and the suppression of this defect by a second mutation in gene 39, coding for one of the three protein chains of T4 DNA topoisomerase. In this study we examined these effects at the protein level. On infection by nrdB93 not only was the beta 93 protein chain altered, as shown by its migration relative to the wild type protein in electrophoretic gels and by its temperature sensitivity, but the infected cells showed very low levels of the protein. However, on infection with the double mutant of nrdB93 and 39-01 (gene 39) the concentration of beta 93 chain returned to the values of beta protein found with wild type phage. A double mutant bearing nrdB93 and an amber mutation of gene 39 also suppressed the nrdB93 defect. By contrast, a temperature-sensitive mutant of gene 39, A41, did not show suppression at either 30 or 41 degrees C. Amber mutations in the two other genes coding for T4 DNA topoisomerase, 52 and 60, did not suppress the defect. We propose that the deficiency in the quantity of beta 93 chain and the suppression of this defect occur at the transcriptional or translational expression of the nrdB93 gene and that a specific domain of the gene 39 protein, not acting in the capacity of T4 DNA topoisomerase, inhibits the expression.