This study carried out to evaluate the effect of bitter vetch seeds on serum proteins and biochemical parameters in broiler chickens. A total of 1320 one-day old broiler chicks of a commercial breed were placed in 64 pens. Treatments were included raw and four different processed bitter vetch seeds in three levels (150, 300 and 450 g kg-1) and a corn-soybean based diet as control. Each treatment group consisted of four replicates. Processing methods were included soaked in water for 12 h, autoclaved, then dried at room temperature (SAD); ground, soaked in water for 24 h, autoclaved and dried (GSAD); ground, soaked in water for 47 h with exchange water every 12 h, cooked and dried (GSCD) and ground, soaked at 1% acetic acid solution for 24 h at 60°C (AA). Feeding raw, AA and GSAD seeds decreased serum albumin significantly (p<0.05) in 21 days old chicks. Chickens that fed with raw and treated bitter vetch seed had lower α1 and γ globulins than control (p<0.05). Increasing raw and treated bitter vetch seeds from 15 to 30 and 45% decreased albumin, α1 and γ globulins and increased α2 and β globulins significantly (p<0.05). In 14 days old chicks feeding raw and treated biter vetch had no effect on serum urea, but uric acid concentration decreased significantly (p<0.05). Feeding SAD seeds increased serum urea significantly (p<0.05), but uric acid concentration did not change with feeding raw and treated bitter vetch seeds in 42 day old chicks. Adding raw and treated bitter vetch seeds to diet increased T4 and decreased T3 concentrations in all ages. At 28 days old chicks, feeding raw and treated biter vetch seeds decreased alkaline phosphatase concentration significantly than control. Results showed that raw bitter vetch seeds have some toxic effects on metabolism in broiler chickens and GSCD and SAD treatments were more effective to detoxification of this seed
