High-titer antibody against HTLV-I in carrier mothers is proposed as a secondary parameter for risk of viral transmission to the children via milk. For titration of antibodies, a less expensive modified gelatin particle agglutination assay (approximately 40% of the standard cost) was developed. None of 11 carrier mothers with antibody titers of less than 4000 had carrier children, whereas 11 out of 17 mothers with titers of 256,000 or higher had carrier children. The antibody titer was correlated with antigen-bearing cells detectable in cultures of peripheral blood T-lymphocytes, which was previously described as a marker of risk for transferring HTLV-I to children.