Abstract. Adebayo OC, Ogidi CO, Akinyele BJ. 2019. Nutritional value and safety of castor bean (Ricinus communis) seed detoxified in solid-state fermentation by Pleurotus ostreatus. Biofarmasi J Nat Prod Biochem 17: 51-60. The nutrient and antinutrient contents of unfermented castor seeds (UCS), castor seeds fermented with Pleurotus ostreatus (CPF) and “Ogiri,” a naturally fermented condiment from castor seeds (CSF) were determined. Proximate analyses of all the samples were carried out using methods of Association of Official Agricultural Chemists. The biosafety of the samples was carried out using the animal model. The raw castor seeds (UCS) had the highest carbohydrate (61.04%), ash (6.02%), fats (6.65%), fiber (6.62%), and calcium (0.30 mg/100g). Castor seeds fermented with Pleurotus ostreatus (CPF) had the highest protein content of 20.47%, magnesium of 7.16 mg/100g, alkaloids (7.40 mg/g) and saponins (6.69 mg/g) . CSF had the highest zinc (0.69 mg/100g). CPF had the lowest tannin (0.05 mg/g). All the essential amino acids increased significantly in the fermented samples. Tryptophan was absent in UCS but present in CSF and CPF with values of 0.78 mg/100g and 1.15 mg/100g, respectively. The hematological analysis of the rats fed CPF had the highest WBC of 5.43×109, which indicated a positive immunomodulatory effect. Hence, this study revealed that Pleurotus ostreatus degraded the toxic compounds in castor seeds to a large extent and enhanced the nutritional contents of the final product.