The partial amino acid sequence of the variable region of a rabbit anti-p-azobenzoate antibody was determined. The sequence determination was facilitated by a modified dimethylallylamine sequencer program in which 1-chlorobutane is used as both the S1 and S3 solvents. To establish the primary structure of virtually the entire light-chain variable region, two different sequencer runs were sufficient: a double cleavage run of 66 cycles on the intact light chain and a single cleavage run of 40 cycles on a peptide isolated from a tryptic digest of the succinylated light chain. Within the limits of the methods used, the light chain appears completely homogeneous. The heavy chains could be more heterogeneous, since the antibody preparation gives at least 18 bands on isoelectric focusing.