1. Sera from an outbreak of about forty cases of dengue fever due to dengue-1 virus have been studied using the neutralization, haemagglutinin-inhibition, and complement-fixation tests.
2. The neutralization test was the most specific and the complement-fixation test the least so.
3. The neutralization test is essential for clear identification of the causal virus by serological means, and serial serum specimens from each patient must be examined.
4. The haemagglutinin-inhibition test can be used to screen patients in outbreaks where some cases have been fully identified by neutralization tests.
5. Homologous neutralizing antibodies persist in high titre for at least 30 weeks after infection, while heterologous antibodies have disappeared by that time.
6. Both haemagglutinin-inhibiting and complement-fixing antibodies to homologous and heterologous viruses usually persist for at least 30 weeks, although the homologous titres tend to be highest.
7. The implications of these findings in serological surveys are discussed.
8. Some evidence suggesting the occurrence of inapparent infections during the epidemic is presented.
I am greatly indebted to my staff for help with this work: especially to my senior technician, Che Ali bin Mohamed Amin, and to Che Mohamed bin Omar who drew the figures.