R forms of Pneumococcus may be converted into S forms of the homologous Type. In addition to the methods previously reported,—(1) animal passage and (2) growth in anti-R sera,—conversion may be effected by the following procedures as employed by Griffith; (1) The subcutaneous injection, in white mice, of large amounts of living R organisms. (2) The subcutaneous injection, in white mice, of small amounts of living R organisms together with the heat-killed bacteria from large amounts of homologous S cultures. There are "varying degrees of constancy of the R variant"; but by these means it has been possible to effect conversion of all R forms selected. Attempts to cause a further "degradation" of R organisms by continued growth in homologous immune serum have been unsuccessful.

Type II S and III S vaccines are equally effective in producing conversion when heated for 15' at 60°C., or for 15' at 100°C. Type I S vaccine, however, while effective in causing conversion when heated for 15' at 60°C., apparently loses this property when heated for 15' at 100°C.

R vaccines, and vaccines of other organisms, when injected together with live R cultures, have always failed to produce conversion.

The causes responsible for conversion under these experimental conditions are discussed and the possibility of the occurrence of a similar process under natural conditions in human beings is indicated.

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