Infection of breeder flocks in China with subgroup J avian leukosis virus (ALV-J) has increased recently. In this study, we have developed a loop-mediated isothermal amplification (LAMP) assay for rapid detection of ALV-J from culture isolates and clinical samples. The ALV-J-specific LAMP assay efficiently amplified the target gene within 45 min at 63°C using only a simple laboratory water bath. To determine the specificity of the LAMP assay, various subgroup ALVs and other related viruses were detected. A ladder pattern on gel electrophoresis was observed for ALV-J isolates but not for other viruses. To evaluate the sensitivities of the LAMP assay and conventional PCR, the NX0101 isolate plasmid DNA was amplified by them. The detection limit of the LAMP assay was 5 target gene copies/reaction, which was up to 20 times higher than that of conventional PCR. To evaluate the application of the LAMP assay for detection of ALV-J in clinical samples, 49 samples suspected of ALV infection from breeder flocks were tested by the LAMP assay and PCR. Moreover, virus isolation from these samples was also performed using cell culture. The positive-sample ratios were 21/49 (43%) by conventional PCR, 26/49 (53%) by the LAMP assay, and 19/46 (41%) by virus isolation. Additionally, a positive LAMP reaction can be visually ascertained by the observation of turbidity or a color change after addition of SYBR green I dye. Consequently, the LAMP assay is a simple, rapid, and sensitive diagnostic method and can potentially be developed for rapid detection of ALV-J infection in the field.Avian leukosis viruses (ALVs), which belong to the Alpharetrovirus genus of the family Retroviridae, can induce transmissible benign and malignant neoplasms in poultry (13, 24). ALVs are divided into subgroups based primarily on virus neutralization, their host range, and viral interference patterns (4, 5, 13). The envelope glycoprotein (gp85) is responsible for subgroup specificity. In general, the endogenous nonpathogenic subgroup E viruses are present in nearly all chicken lines (13,26). Subgroup C and D viruses are rarely seen in the field. The exogenous viruses (subgroups A, B, and J) mainly induce lymphoid leukosis (LL) and myeloid leukosis (ML) in field flocks (13). In the late 1980s, the prototype strain of subgroup J avian leukosis virus (ALV-J), HPRS-103, was first isolated from commercial meat-type chickens in the United Kingdom (22). ALV-J infection subsequently caused heavy losses worldwide in broiler breeder stocks. Many strains of ALV-J have been reported worldwide in the past 20 years (2,11,30). Sequence analysis has demonstrated that the gp85 genes of subgroups A to E are closely related, sharing around 85% nucleotide sequence identity with each other. However, the HPRS-103 gp85 gene shares only 40% identity with members of other subgroups. The gp85 genes of endogenous avian retroviral sequence (EAV-HP) elements share over 97% identity with that of ALV-J; thus, it was hypothesized that the new avian patho-
