A new gelatin particle agglutination test was developed for assay of natural antibodies to adult T-cell leukemia virus (ATLV/HTLV-I). Partially purified viral antigen from culture fluid of a virus-producer cell line was coated on artificial gelatin particle carriers. A high correlation was observed between the titers of antibodies determined by the agglutination test and by indirect immunofluorescence assay. The agglutination test is simple, sensitive and specific, and should be useful for mass screening of human sera for viral antibodies.