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The trace elements are so called because they constitute less than 0.01% of the weight of the human body; however, despite their relative scarcity, their atoms are present in large numbers and each is believed to play an important role in human growth and development. For example, chromium is perhaps one of the least abundant trace elements in the body and yet even a single hepatocyte contains about 4.5 million atoms of this element. The name "trace element" is, of course, quite arbitrary and survives from the time when early investigators were experiencing great difficulties in measuring these substances; the introduction of more sophisticated techniques has resulted in an enormous increase in our understanding of the role these elements play in metabolism. This review deals with zinc, copper, manganese, selenium, and chromium. The amount of space devoted to each element is not intended to correspond to its importance in metabolism

In the course of carrier studies on infants and on cultures isolated from infected persons in the same age group, a large number of strains classified as group D (Lancefield) were encountered. The frequency with which these strains were encountered, together with the relative scarcity of work done on this serologic group as compared with that on the Lancefield group A (in which the majority of organisms causing human infections are placed) seemed to justify analysis. The present interest in group D was further stimulated by the isolation of a strain of streptococci, originally misclassified as group E in another laboratory, from an infant with an acute, fulminating infection. It was not until rabbits had been immunized with this strain and reciprocal absorption tests had been done with the resulting serum and with known strains of the various serologic groups that the correct identity of this strain was established. The