The detection of immunoglobulin (Ig)G and IgM antibodies to dengue 1 virus was studied by a simple enzyme immunoassay, in which infected cultured cells infected with dengue virus were used as antigen (EIA-ICC). Detection of anti-dengue 1 IgG by EIA-ICC was correlated with haemagglutination assays. EIA-ICC anti-dengue 1 IgM detection was less sensitive than IgM capture enzyme-linked immunosorbent assay. IgG and IgM responses in dengue 1 infection were studied by EIA-ICC, using sera collected at different intervals after onset of illness: IgM and IgG appeared on the 4th day of disease; the highest IgM mean titres were detected on the 7th day and IgM was not detected in sera obtained after the 60th day; the highest mean titres of anti-dengue 1 IgG were seen in sera obtained between 22 and 30 d after onset of illness. EIA-ICCs for 6 flaviviruses and 1 alphavirus were conducted with sera from patients infected with dengue 1, and primary and secondary infections of other flaviviruses. The results showed that anti-dengue 1 IgG detection was sensitive, and the antibodies were cross-reactive among the flaviviruses. Anti-dengue 1 IgM detected in dengue 1 patients was mostly type specific. The pattern of secondary dengue infection, i.e. the presence of IgG and a low titre or absence of IgM antibodies, was observed in the sera of 6 patients obtained in the first week after onset of illness. EIA-ICC is useful for dengue diagnosis, surveillance and sero-epidemiological studies.