An in vitro quantal assay (TCID50) for a non-occluded baculo-like virus isolate from naturally infected Penaeus japonicus obtained from China and experimentally infected P. stylirostris was developed using primary shrimp lymphoid cell cultures in Primaria 24-well tissue culture plates. The virus caused cytopathogenic effect (CPE) in the cell cultures as early as 2 day post-infection (p.i.). Initially, the cells rounded up and finally detached from the culture vessel as the infection progressed. At the present time, there is no established quantitative in vitro cell culture protocol for the assay of this baculo-like virus which has been reported by our laboratory to be highly pathogenic for P. stylirostris and P. vannamei, the two species of penaeid shrimp commercially cultured in Hawaii and the Western hemisphere. This quantal assay thus provides a simple and convenient method for the detection and assay of infectious virus in cultured penaeid shrimp.