Spheres of influence: no POPS. Persistent organic pollutants.

(ONFLICTING results have beeni reported with neonatal thymectomy in relation to several different chemical carcinogens given to animals by various routes. Some authors (Miller et al., 1963; Grant & Miller, 1965; Nomoto & Takeya, 1969) have shown that tumour induction could be increased in thymectomized animals when they were compared with a group that had had sham thymectomy. When thymectomized and surgically intact animals were compared, other authors (Allison & Taylor, 1967; Law, 1965) found no significant difference in tumour induction. However, Nishizuka et al. (1965) have shown a higher incidence of hepatic tumours in thymectomized animals than in intact animals, but comment that their results might in fact be due to faster tumour growth. Johnson (1968a,b) showed that the effect of thymectomy versus intact controls was to shorten the latent period of tumour induction, but that the growth rate was unaffected. In contrast to Johnson, Balner & Dersjant (1966) reported negative results when comparing 3 groups, thymectomized, sham thymectomized, and intact controls, although all 3 groups also received an allogeneic skin graft. Johnson (1 968a) commented that this might be a "sufficient non-specific stimulation of the host's defective immunological defences to counteract the effects of thymectomy". Results of experiments carried out by Yasuhira (1969) and by Polliack et al., (1972) (to be discussed later) were also partly explained by neonatal surgical intervenAcceptecl 27 November 1978