Lepromin reactions of leprosy patients in the mainland and Okinawa of Japan were examined by using the standard (160 million bacilli/ml) and diluted lepromins, for the pur-pose of establishing practical use of the latter and understanding immunological character-istics of leprosy in Okinawa. The size of reactions expressed by mm scale were con-verted by common logarithms for statistical evaluation, because the distribution curves shown in Fig. 1-3 were approximately equal to a distorted shape of normal distribution. The results were as follow.
1. Both Fernandez' and Mitsuda's reactions to the lepromins diluted in 1:4 or more were significantly weaker than those to the standard, as demonstrated by the percentage of completely negative (0-1mm) reactions (Tables 1 and 2) and by the difference between two reactions in both forearms of the same patient (Tables 7 and 8).
2. Average size of reactions of the patients in Okinawa was significantly larger than those in the mainland, irrespective of dilution degree of the lepromins (Tables 4 and 5).
3. A correlation coefficient was significant between the Fernandez' and Mitsuda's reactions to each of the standard, 1:4 and 1:8 diluted lepromins (Table 9).
4. Normal human, autoclaved skin suspension caused the cutaneous response resembling Fernandez' reaction in a significant number of the patients, but the most of their responses disappeared completely after 4 weeks and a mean size of reactions in a few cases was significantly smaller than that of Mitsuda's reactions (Tables 3, 6 and 7).
5. Significant fall in the percentage of completely negative reactions and significant increase in the mean of 2mm or larger sizes of ractions were observed in the patients with lepromatous leprosy, according to three stages which were classified as progressive, retrogressive and quiescent in this order (Table 10).
6. A cut-off point of positive Fernandez' reactions was proposed to be 8mm, according to different criteria for positive reactions in lepromatous cases as well as in non-lepro-matous. On the other hand, Mitsuda's reactions should rather be recorded by the size of reaction which could be used as an indicator of cell-mediated immunity against Mycobacterium leprae.