A Caucasian French family was investigated including two Jk(a-b-) sibs who had developed anti-JkaJkb. The parents were first cousins. Quantitative study of Kidd antigens using titration scoring and HD50 assay clearly showed the parents and a maternal aunt, phenotypically Jk(a+b-), to have a weak expression of Jka documenting thereby the transmission of the postulated silent gene Jk. Absorption, elution and coagglutination assays revealed the cross reactivity of the antibody developed by the two sibs. Leucocytes and platelets from these two individuals were not shown to absorb anti-JkaJkb.