To obtain greater insight into the interaction of Ca, citrate and phosphate ions with casein, 31P NMR measurements were performed on combinations of these ions with αs- and κ-caseins. It was found that addition of αs-casein to a Ca phosphate solution in D2O at 27 °C and pD 6·4 resulted in a downfield shift of the 31P singlet. An almost identical shift was observed with κ-casein, but no shift was found when only phosphate ions were present or when Ca2+ were added to phosphate ions in the absence of casein. Separate experiments with poly-l-lysine, mol. wt approx. 35000, resulted in similar downfield 31P chemical shifts of Ca phosphate as with both caseins, whereas no shift was observed when poly-glycine was added. From these results it can be concluded that Ca and phosphate ions associate with casein in a co-operative manner, probably in the way described by ter Horst (1963) as a complex with the NH3+-groups of lysine or arginine in a structure such as: (casein—NH3+)—P043-—Ca2+. The formation of this complex may be enhanced by citrate ions, since preliminary results have shown that addition of Na citrate to a solution of αs-casein with added Ca phosphate produces a broadening of the 31P signal as well as a chemical shift.