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A bioassay study was conducted to investigate the effects of substituting casein with graded levels of detoxified Jatropha curcas seed cake protein isolates (JPI) as a protein source on the growth performance, feed efficiency ratio (FER) and its protein values using rats as an animal model. Thirty 21-day-old male Sprague–Dawley weaned rats were randomly divided into 5 groups, each group with 6 replications (n = 1). Each group consumed one of the following diets: protein-free, casein (CAS) and JPI diets (JPI20, JPI40 and JPI60; different levels of JPI to replace the casein at concentrations of 20%, 40% and 60% on crude protein basis). Feed intake and protein intake showed no difference among the rats fed JPI20, JPI40 and CAS diets (P > 0.05). However, these parameters were lower in the rats fed JPI60 than in rats fed CAS (P < 0.05). The rats fed diets containing JPI had lower body weight gain, protein efficiency ratio and net protein retention than those fed CAS diet (P < 0.05). When the level of JPI used to replace the casein was lower than 40%, protein efficiency ratio (PER) was close to or higher than 2.0, which suggests that JPI could be viewed as a high-quality protein. Inclusion of JPI in the diet decreased alkaline phosphatase activity. The values were significantly lower in rats fed JPI20 and JPI40 than in rats fed CAS (P < 0.05). No histopathological changes were observed in livers and kidneys in the rats fed JPI diets. The results demonstrate that JPI could be used as an efficient protein source at a level of no more than 40% of dietary protein source. Keywords: Jatropha curcas, Seed cake protein isolates, Protein efficiency ratio, Growth performance, Rats