An indirect enzyme linked immunosorbent assay (ELISA) using monoclonal antibody (MAb) originated from the native Thai isolates of P. vivax (McPV1) and the polyclonal antibody (PAb) raised against Nepali isolates of P. vivax was developed for detection of P vivax antigens in red cell lysates. The assay was specific (100%) since it was positive only with P. vivax-infected erythrocytes and was negative when erythrocytes from 40 healthy individuals from malaria non-endemic areas and 40 P. falciparum infected erythrocytes were tested. When the assay was applied to 203 vivax blood samples already proven by microscopic examination collected from Dhanusha district of Nepal, and using the cut-off level of the mean optical density (OD) (0.144) of 40 healthy individuals who had been living in malaria-endemic areas (0.073) + 2 SD (0.016), the assay could detect 189/203 samples, indicating the sensitivity of the test was 93.1% with a detection limit of erythrocytes of 240 parasites/10(6) erythrocytes. In addition, the assay was negative when 40 blood samples with fever of unknown origin, collected from the same malaria-endemic areas, were tested. However, there was a significant correlation between OD values and parasitemia (r=0.649; p=0.018). The results indicate that MAb-PAb indirect ELISA using MAb raised against Thai isolates of P. vivax as the coating antibodies, and polyclonal antibodies raised against local Nepali isolates as the detecting antibody, could detect P. vivax antigens with high degrees of sensitivity and specificity. Furthermore, it seems that the McPV1 MAb raised against Thai isolates of P. vivax could recognize the antigens of Nepali isolates in a wide range of blood samples.