Several HSV-1 proteins including the major capsid protein (VP5), two minor capsid proteins (VP11-12 and VP18.8), the alkaline nuclease and glycoprotein gH have been reported to be encoded by the left-most one-third of HSV-1 UL DNA. In this paper, we present physical mapping data and phenotypic analysis of six ts mutants whose mutations lie within this region and which collectively represent four functional complementation groups (1-6, 1-7, 1-10, and 1-26). In this study, mutants in complementation group 1-10 were found to be defective in the synthesis of viral DNA, late viral polypeptides, and the formation of mature capsid-like structures--properties characteristic of other ts mutants defective in functions required for viral DNA synthesis. Two DNA-positive mutants in complementation group 1-7 fail to induce capsid formation and probably possess mutations in coding sequences for VP5. Mutants in two other complementation groups (1-6 and 1-26) synthesize significant levels of viral DNA, late polypeptides, and capsids. The functions of the gene products represented by these mutants remain to be determined.