a b s t r a c t : The pH-dependent change in conformation o f bovine serum albumin, located between pH 6 and 9 (neutral transition), was studied by means o f optical rotation measurements at 313 nm and hydrogen ion titration experiments, both in the presence o f KC1 or CaClj. The optical rotatory dispersion measurements revealed that with CaCl2 the transition proceeds at lower pH values and within a smaller pH range than without calcium. From an analysis o f the titration curves, combined with the observed influence o f calcium ions on the neutral transition, it could be concluded that the transition causes a p K shift o f certain groups, probably imidazole groups,