Another method to add to the several which you will hear about this evening for propagation of Tacaribe and Junín viruses has been the development of a plaque assay system in rhesus monkey kidney (RhMK) tissue cultures. This brief report will include a description of this plaque method and its applicability to studies of virus multiplication in various hosts, antigenic relationships between the two viruses, antibody measurements, and virus pathogenesis in laboratory animals. Plaque assays were performed using the Hsiung-Melnick overlay supplemented with lactalbumin hydrolysate and yeast extract as recommended by Coleman. Tacaribe virus plaques first appeared after 6 to 8 days as small, circular, clear areas in the monolayer, 1–2 mm in diameter. They were readily visible when the bottle culture was held against a white background. Plaques increased in size upon continued incubation and became 5 or more millimeters in diameter. The plaques could be counted by direct observation usually from day 7 or 8 (Fig. 1).