In this study, the performance of IgG and IgG1 anti-HTLV-1 reactivity obtained by a flow cytometric assay was evaluated to verify its applicability for the diagnosis of persons infected with HTLV-1, including asymptomatic carriers and patients with myelopathy. The ability to identify patients with myelopathy among persons infected with HTLV-1 was also examined. Western blot assays were performed to assess the reactivity profiles of sera from asymptomatic carriers and patients with myelopathy against viral proteins. The data showed that IgG1 detected by flow cytometric assay is effective for the diagnosis of persons infected with HTLV-1 with 97% sensitivity and 100% specificity. IgG and IgG1 exhibited high performance in distinguishing patients with myelopathy from asymptomatic carriers. Using serum dilutions and cut-off points established previously a second HTLV-1 carrier group was tested using flow cytometric assay to detect IgG and IgG1. The data demonstrated sensitivity of 93% and 98%, respectively, confirming the high reactivity of persons infected with HTLV-1 detected by this method. Western blot assays confirmed the high specificity of MT-2 cells as a reliable source of viral antigen since only sera from persons infected with HTLV-1 recognised MT-2 proteins. Furthermore, a high reactivity to Gag and Env proteins was observed, especially among patients with myelopathy. These data suggest that flow cytometric detection of IgG1 is a valuable, non-conventional serological method to diagnose HTLV-1 infection and for research purposes.