The onset and duration of resistance in experimental survivors of Penaeus japonicus produced by an intramuscular injection with white spot syndrome virus (WSSV) were surveyed by re-challenge tests with the virus conducted at weeks 1-4 and months 1-3 post initial exposure (PIE) to the virus. Virus neutralising activity in the survivors' plasma was also examined. Plasma-treated WSSV was separated from the plasma by centrifugation and then injected into naïve shrimp, in parallel with each re-challenge test. Re-challenge tests of the survivors conducted at weeks 1-4 PIE revealed that the resistance commenced at week 3 (relative percent survival, RPS: 39%) and almost fully developed at week 4 (RPS: 58%), because statistically significant differences in survival rates were observed between the test (previously virus exposed) and control groups at weeks 3 and 4. Re-challenge at months 1-3 PIE resulted in RPS values of 67, 54 and 6%, respectively, indicating the resistance persisted until month 2. RPS values in neutralisation tests performed at weeks 1-4 and months 1-3 PIE were -5, 14, 36, 50, 100, 38 and 6%, respectively, which coincided with the RPS values in each re-challenge test conducted in parallel. The present results demonstrated that resistance of P. japonicus against the viral pathogen developed 3 or 4 weeks after an exposure to the virus, and it persisted for another month at 24 degrees C. The resistance was paralleled by a humoral neutralising factor(s) in the plasma of shrimp.