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Parasporal inclusion proteins from a total of 1744 Bacillus thuringiensis strains, consisting of 1700 Japanese isolates and 44 reference type strains of existing H serovars, were screened for cytocidal activity against human leukaemia T cells and haemolytic activity against sheep erythrocytes. Of 1684 B. thuringiensis strains having no haemolytic activity, 42 exhibited in vitro cytotoxicity against leukaemia T cells. These non‐haemolytic but leukaemia cell‐toxic strains belonged to several H‐serovars including dakota, neoleonensis, shandongiensis, coreanensis and other unidentified serogroups. Purified parasporal inclusions of the three selected strains, designated 84‐HS‐1‐11, 89‐T‐26‐17 and 90‐F‐45‐14, exhibited no haemolytic activity and no insecticidal activity against dipteran and lepidopteran insects, but were highly cytocidal against leukaemia T cells and other human cancer cells, showing different toxicity spectra and varied activity levels. Furthermore, the proteins from 84‐HS‐1‐11 and 89‐T‐26‐17 were able to discriminate between leukaemia and normal T cells, specifically killing the former cells. These findings may lead to the use of B. thuringiensis inclusion proteins for medical purposes.

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