The partial amino acid sequence of pooled, microheterogeneous pig immunoglobulin lambda chains was determined previously (Franĕk, F. (1970), FEBS Lett. 8, 269; Novotný, J., and Franĕk, F. (1975), FEBS Lett. 58, 24). In the present study, citraconylated pig lambda chains were digested by trypsin under conditions in which some of the epsilon-amino groups of lysine residues unmask. The resulting fragments were purified by gel filtration and ion-exchange chromatography at pH 3.0 in buffers containing urea; some of the fragments were found to be of intermediate size (i.e., larger than normal tryptic peptides but smaller than "citraconyl" peptides), thus permitting overlap information and amino acid sequences of all the 14 tryptic peptides to be deduced from amino acid compositions and partial amino acid sequences of selected fragments. In addition to completing the major amino acid sequence of pig immunoglobulin lambda chains, the present study demonstrates that it is possible to sequence microheterogeneous proteins with a suitable fragmentation strategy.