103 Cynomolgus monkeys were tested for the serum antibody of STLV-1 by Gelatin particle agglutination (PA), Immunofluorescence assay (IFA) and Western blot (WB).The results showed that 13 WB-positive sera reacted in the PA and IFA; A positive reaction,however,was detected in 3 and 1 out of 90 WB-negative specimens by PA and IFA.