Many studies on prophylactic immunization against pneumococcal pneumonia have been made, using a number of different antigenic preparations. Almost all investigators have concluded that immunization exerts a beneficial effect. In most of the studies, however, certain variables have clouded interpretation of the results. Among the variables, the following appear to be of greatest moment: differences in the composition of the immunized and control groups; uncertainty as to whether the specific pneumococcal types included in the immunizing preparation were the same as those currently causing pneumonia; failure to determine whether the observed decline in cases in the immunized group was due to a decrease in cases caused by the pneumococcal types included in the vaccine; inadequate control of the antigenicity of the preparations used.The subject of antipneumococcal immunization has been reviewed recently by Heffron (1) and references will be made in the present paper to certain aspects only.The studies of Lister and Ordman (2) among native laborers in South Mrican mines between 1930 and 1934 suggest that immunization with a polyvalent pneumococcal vaccine reduces the incidence of pneumonia caused by the same pneumococcal types.

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In an earlier paper (1) a study was reported of the antibody content, in weight'units, of the sera of human volunteers at varying intervals from 2 weeks to 2 years after injection of pneumococci or of the specific polysaccharides of two or three pneumococcal types. On the basis of this study and of earlier, less precise investigations an experiment was undertaken looking toward immunization against Types I, II, V, and VII pneumococci of incoming trainees at an army camp at which lobar pneumonia was endemic. The epidemiological results of subcutaneous injection of trainees with the specific polysaccharides of Types I, II, V, and VII pneumococci have been published (2). Analyses of ser~ of randomly selected subjects who had participated in this experiment have now been completed, and the data are submitted herewith. EXPERIMENTALThe sera of two groups of subjects were analyzed: Group 1: Randomly selected trainees (roughly 1 out of every 300) were bled to the extent of 30 to 50 ml. (0 bleeding) and then injected subcutaneously with 1.5 ml. of a solution containing about 0.03 mg. each of the specific polysaccharides of Types I, II, V, and VII pneumococci per ml. About 2 months later as many as possible of these men were again bled (bleeding 1). Two bleedings were obtained and analyses were completed in nineteen instances. Group 2: Bleedings were collected similarly, but the subjects were injected with 1.5 ml. of salt solution. The sera of this group were studied in order to determine whether or not such exposure to pneumococcal infection as might have occurred in normal contacts during the 2 months' period was sufficient to induce the formation of measurable antibodies to the four pneumococeal types studied. This group, therefore, served as an analytical control to the other group. Analyses were completed in ten instances in group 2.All analyses were carried out according to (3) as given in detail in (1). As previously emphasized, it is usually necessary to allow each set of analyses to stand 8 days in the cold to complete the separation of the small amounts of specific precipitate ordinarily formed. If