Broiler chickens reared from 3 to 8 weeks of age were fed per os on extract or, differing in chemical composition fractions of the extract (A₀-A₆) from bitter lupin (L. angustifolius var. Mirela) seeds (Experiment 1), the extract and its selected fractions (A₁, A₃, A₅) from the seeds of Mirela as well as extract from sweet lupin var. Emir (Experiment 2). The extracts from bitter lupin harvested in two consecutive years, used in the experiments, differed in alkaloid content. The extracts and fractions did not negatively affect growth, feed intake or health of the chickens. Only fraction A₅ (alkaloid hydrochlorides) used in Experiment 2, which was from extract with higher alkaloid content, reduced (P<0.01) body weight gain of the chickens. The T₃ level in the peripheral blood serum was the lowest in the birds receiving fractions containing sugars (A₃) and the highest in the group receiving alkaloid hydrochlorides (A₅). The T₄ level was the highest in the chickens receiving extract from sweet lupin. The extracts and fractions had no effect on the red blood cell count while leukograms showed only undirected variation, except for basophil and eosinophil in blood of which level was higher in experimental than control birds.