IN a communication under the above title, published in Nature of May 31, 1947, p. 746, J. L. Mongar and A. Wassermann report some experiments on the de-swelling of calcium alginate fibres in salt solutions. They noted that water-swollen alginate fibres when placed in various alkali-salt solutions first contract and then increase in length. The authors suggest that the contraction results from destruction of calcium ion cross-links between alginate chains as the calcium is replaced by alkali ions. They propose also that plastic flow is mainly responsible for the final swelling effect.