An improved ELISA for the detection of antibodies to ovine and caprine lentiviruses has been developed. The assay employs two monoclonal antibodies (Mabs) directed against the major core protein (p28) of the virus in a modified double antibody sandwich blocking procedure. The modification was necessary to circumvent the consequences of using low affinity Mabs and to lower the chance of false-negative results. A novel one-step concept was developed in which washing between steps was largely omitted. The new assay was designated complex trapping blocking (CTB)-ELISA. A range of sera from sheep and goats was tested comparatively in CTB-ELISA, in an indirect ELISA and in an agar gel precipitation test. The CTB-ELISA proved sensitive and highly specific and in addition reliable and easy to perform.