The complete amino acid sequence of the lambda light chain and the variable domain of the heavy chain of an anti-Rh(c) human monoclonal antibody were determined. The lambda chain presents a long third complementarity-determining region sequence with unusual amino acid insertions at the C-terminus. The proposed sequence indicates that this lambda chain may be assigned to the variable region subgroup I. The J segment is identical to that of J lambda 2 except for the first amino acid residue. Positions 152 (serine) and 190 (arginine) from this sequence correspond to the Kern-Oz- isotype, respectively. The VH segment can be classified as a VHIII subgroup member. The CDR1 segment of the anti-Rh(c) VH region has the same sequence as the VH of human BRO protein except for the first residue of the CDR1. The amino acid sequence of the anti-Rh(c) D segment does not match any published D segment. The JH segment used in this protein can be classified as a JH3 with a single amino acid difference at the fourth residue.