CHRONIC pulmonary disease (Weber and Engelhardt, 1933) and contact dermatitis (DeNardi, Van Ordstrand and Carmody, 1949) have been observed in many beryllium workers. An immunological mechanism was first suggested (Sterner and Eisenbud, 1951) because only a small percentage of exposed workers developed chronic lesions, with an insidious onset several months after exposure. The histological picture of the granulomatous lesions was characteristic. Sterner and Eisenbud's hypothesis was supported by the fact that the cutaneous reactions to soluble beryllium salts of the delayed hypersensitivity type (Curtis, 195 1) were seen in patients suffering from berylliosis, and granuloma formation in people who were cut by beryllium phosphor-coated glass from broken lamps (Dutra, 1951). Furthermore, beryllium fluoride is an excellent sensitiser in healthy man (Curtis; McCord, 1951).Similar responses have been produced experimentally in animals. Chiappino, Cirla and Vigliani (1969) demonstrated that after endotracheal injection of beryllium oxide (BeO) a lung reaction characteristic of hypersensitivity developed in some guinea-pigs, and that these animals also had dermal hypersensitivity to soluble salts. Furthermore, the application of the sulphate (BeSO4) to the ear induced the same histological changes in the regional lymph-node as known sensitisers. A state of delayed hypersensitivity has also been induced with various salts of beryllium given by different routes (Belman, 1957(Belman, , 1969Alekseeva, 1966; Pol&, Barnes and Turk, 1968;Chiappino et al., 1969). These findings suggest that the tissue reactions induced by beryllium are immunological ones of the delayed hypersensitivity type. This was confirmed in guinea-pigs by passive transfer by means of lymphoid cells (Cirla, Barbiano di Belgiojoso and Chiappino, 1968), and by the inhibition of the intradermal reaction to beryllium by antilymphocytic serum (Chiappino, Barbiano di Belgiojoso and Cirla, 1968).The work reported here shows that immunological paralysis (or tolerance) to beryllium can be readily induced in adult guinea-pigs, and can be recognised by inhibition of the delayed skin-hypersensitivity reactions to the topical application of beryllium in pre-treated animals. Beryllium compounds may induce either sensitisation or tolerance. Beryllium as the citrate is tolerogenic, whereas a compound more easily phagocytosed or one that reacts with proteins is immunogenic. This supports the conclusions previously drawn by Macher and Chase (1969a) from the results of their work on the elimination from the skin of