The plaque reduction neutralization test (PRNT) has been used routinely in serological studies with such arenaviruses as Junin, Machupo, and Parana. However, difficulties have been encountered in using the PRNT for LCM virus, while conflicting views have been expressed about the reliability and efficacy of the test with Lassa virus. We have therefore investigated and evaluated the plaque assay for Lassa virus. In addition, the suitability of the PRNT for determining the potency of a serum and its efficacy in passive immunization for the treatment of Lassa fever was also investigated. The Lassa virus plaque assay satisfied the criteria proposed by Cooper  for determining satisfactory plaque technique. Lassa virus plaques appear within 3 days of inoculating Vero cell cultures. By day 5, the plaques are clearly defined, discrete, and measure 1.5 to 2.0 mm. In the plaque reduction neutralization test, the use of native non-inactivated serum was required for a reliable and reproducible determination of serum antibody titer. The potency and suitability of a serum for Lassa fever serotherapy was determined by the use of a constant serum-varying virus (CS-VV) and/or a constant virus-varying serum (CV-VS) PRN technique.