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1. Highly purified rabbit Type III pneumococcus anticarbohydrate proved to be homogeneous in the ultracentrifuge and its sedimentation constant, 7.0·10–13, did not differ from that of the principal component of normal rabbit globulin or of immune rabbit globulin containing up to 50 per cent of anti-egg albumin. The molecular weight of antibody in the rabbit is therefore probably very close to that of the principal normal globulin component, namely, 150,000. 2. Highly purified horse Type I pneumococcus anticarbohydrate, on the other hand, was only homogeneous in the ultracentrifuge when prepared from sera stored without preservative. Its sedimentation constant, 18.4·10–13, coincided with that of the principal globulin component in most of the Felton solutions and purified antibody solutions studied. The molecular weight of pneumococcus anticarbohydrate in the horse is probably three to four times that of the principal normal globulin component. 3. The significance of the differences between pneumococcus anticarbohydrate formed in the rabbit and in the horse is discussed. 4. Results are given of ultracentrifuge studies on the molecular species in solutions of egg albumin-anti-egg albumin specific precipitates dissolved in excess egg albumin. The implications of the results are discussed.

It has long been known that there is some difference between the sera (and plasma) from the new-born animal and that from the adult. In the former case the serum contains less globulin than in the latter. According to earlier investigations by, among others, Howe (2), the proportions between the various globulin fractions are different in the two instances. A few years ago some publications appeared dealing with the electrophoresis of serum from new-born and young animals. They all show the presence of large amounts of a-globulin, small amounts of /3-globulin, and the absence of -globulin in the sera of the new-born calf (1, 3) and foal (8). It was not, however, until the present author (5, 6, 7) started a series of fractionation experiments on calf serum and studied the fractions obtained in the ultracentrifuge that it was realized that a protein entirely different from the known albumins and globulins was present in these sera.

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