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As B.C.G. vaccination is being given extensively to uninfected children in this country. It has become important for pediatricians, general practitioners and public health people to differentiate natural conversion of response to tubercle bacilli from B.C.G. conversion. The authors have investigated this problem by studying 993 pupils in a primary school in Seoul. In our investigation, the difference between the convesion rate with 1/2,000 O.T. and 1/10,000 O.T. at various intervals after B.C.G. vaccination in the B.C.G.- converted group is statistically significant, whereas in the naturally converted group, the difference between the two groups is not statistically significant. There also is a difference in local reaction between the naturally converted group and the B.C.G.-converted group, although it is hard to distinguish with a single reaction whether it is due to natural conversion or to B.C.G. conversion. From both the results of our study and a some of the previous articles on this matter, the authors conclude that natural conversion to tuberculosis may be differentiated from B.C.G. conversion by using both 1/2,000 O.T. and 1/10,000 O.T. at the same time rather than using 1/2,000 O.T. alone, as practiced until now in this country, in order that the reaction may not be erroneous1y read as a naturally-converted positive reaction always.
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