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The dynamics of the serum concentration of protein 27 (P27) of avian leukosis virus and transforming growth factor β2 (TGF-β2) were compared during the period between 29 and 59 weeks of age in two flocks of broiler chicken breeding stock undergoing outbreaks of severe lymphoid leukosis (LL) associated with persistent high mortality (susceptible) and in another two flocks of breeding stock with the presence of avian leukosis virus in association with low mortality due to LL (resistant). The average mean concentration of serum P27 in the LL-susceptible flocks was significantly higher (p<0.05) than that in the LL-resistant flocks in six out of seven samplings performed at 5-week intervals, between 29 and 59 weeks of age. The peak in the average rise of serum P27 in the LL-resistant flocks (309 pg/ml) was associated with the highest level of TGF-β2 (1282 pg/ml) among all flocks and at all sampling times. The significance of TGF-β2 in inhibition of lymphoid tumour development is discussed.