The circular dichroism (CD) spectra of lectins from Dolichos biflorus, Helix pomatia, Lotus tetragonolobus, Phaseolus vulgaris (erythroagglutinin), Pisum sativum, Sophora japonica, and Ulex europaeus I were analyzed, before and after the conformational transitions which were effected by sodium dodecyl sulfate (SDS) and change of pH. It was found that the pleated sheet conformation dominated in these lectins and that the best estimates of the amounts of the sheets can be made from the positive CD bands centered at 195 . 197 nm. The mean residue ellipticities of these bands in native lectins varied between 1.0 . 10(4) and 1.7 . 10(4) deg . cm2 . dmol-1. The conformation of the lectins was perturbed by SDS, which caused the CD bands related to the tyrosine and tryptophan chromophores to diminish and which also caused the reorganization of the main chains into new forms having approximately 15--40% of alpha-helix. Acid facilitated these transitions. The secondary structure of Ulex europaeus I lectin was resistant to SDS in neutral solutions but reorganization occurred in acid solutions. In Helix pomatia lectin the helices were formed chiefly from the aperiodic segments of the main chain. As structural perturbant, SDS was most effective at relatively high concentrations of 5 . 10(-3) to 5--10(-2) M, i.e. above the critical micelle concentration. Experiments with Pisum sativum and Sophora japonica lectins indicated that SDS below critical micelle concentration did not effect the tertiary structure of the lectins.