The replication of orf virus in bovine testis cells was analysed in one-step growth experiments. Newly replicated viral DNA was detected 4 to 6 hours post-infection (p.i.), accumulated rapidly between 8 and 16 hours p.i. and reached a plateau between 25 and 30 hours p.i. Most virus-induced polypeptides were first detected in a two hour period beginning 10 hours p.i., reached a peak rate of synthesis between 14 and 16 hours p.i., and continued at that rate for at least 10 hours. Host polypeptide synthesis declined to very low levels by 20 hours p.i. From these results, the transition between early and late events appears to occur between 8 and 10 hours p.i. Infectious virus was first detected between 16 and 18 hours p.i. and continued to be produced at a steady rate till 40 hours p.i. Up to 35 polypeptides were detected in SDS-polyacrylamide gels of purified orf virions disrupted in SDS/2-ME. Virions treated with NP40/2-ME were separable into soluble and insoluble components by centrifugation. Some 13 polypeptides were found in the soluble fraction and a polypeptide of molecular weight 38,500 believed to be the basic subunit of the virion surface tubule structure. Little difference was found between polypeptide profiles of five independently isolated NZ orf virus strains.