The association‐dissociation behaviour inTodarodes sagittatushemocyanin has been studied by sedimentation velocity measurements at different pH values and in the absence and presence of a number of anions and cations. In the absence of salts the sedimentation pattern is dominated (about 1002) by a relatively low M.W. species (20 S), while 10 mM CaCl2and increasing pH (above 65) stabilize a larger (30 S) species. The highest aggregation (60 S) state reaches a maximum population (about 70%) at pH15and 100 mM CaCl2as well as at 600 mM NaCl or KC1. Parallel tests of the functional behaviour of the system allow a distinction between tertiary and quaternary effects of the various ions, and reveal, within a complex pattern of interactions, the outstanding role of Cl‐ions as strong enhancers of the.O2affinity. The above data are used as a basic to explain some physiological questions concerning the nutritional habits of these animals.