AbstractVariable region nucleotide sequences and respective translated amino acid sequences for three heavy chains (DB3, 11/32 and 10/8) and two light chains (DB3 and 11/32) of monoclonal mouse IgG1anti‐progesterone antibodies have been determined by primer extension mRNA sequencing. The three VHregions exhibit the same rarely observed VHIX gene family and have>88% homology between them. Two associated light chain sequences are 95% homologous and belong to the VxI group. The N‐terminal twenty two amino acids of the x light chain of the third antibody 10/8 have been determined by automated protein sequencing and are identical to those of 11/32. Thus, these three monoclonal anti‐progesterones derived from separate fusions all use VHIX‐Vx1 gene combi