The 10K gene of simian herpes B virus (SHBV) has been located and the nucleotide sequence determined. Its relationship to homologous genes in other herpes-viruses has been examined. The SHBV 10K gene exhibits a closer relationship to its homologue in HSV-1 than to those in the other viruses studied. Nucleotide sequence identity of 61% was found between the HSV-1 and SHBV 10K genes, and 57% identity was found between the corresponding predicted protein sequences. A comparison of the amino acid sequences of the herpesvirus 10K proteins has revealed a number of conserved features. These are examined in relation to possible functions of the 10K proteins. Implications for evolutionary relationships between SHBV and other herpesviruses are discussed.