Five di †erent recently released Brazilian soybean cultivars (Bays, BR-10, Rio Balsas, Serido and Tropical) were compared for their proximate analyses and presence of antinutritional or toxic factors. As expected, the seeds are rich in proteins, varying from 360É7 to 485É4 g kg~1 Ñour, and they also have a high amount of fat (from 183É0 to 215É3 g kg~1 Ñour). Crude extracts from seeds of Bays, BR-10, Serido and Tropical were highly toxic to mice within 1È12 h, depending on the administration route (intraperitoneal, intramuscular or subcutaneous) and dose used while Rio Balsas was not. These acute e †ects were very similar to those produced by the soytoxin, a neurotoxin that has been recently puriÐed from the commercial soybean sold in Brazil. The amount of trypsin inhibited in the presence of crude extracts ranged from 28É5 to 62É5 g kg~1 Ñour. Urease was also present and the seed lectin agglutinated preferentially rabbit erythrocytes. A heat treatment at 92¡C for 1 min destroyed completely the toxic activity while the haemagglutinating and trypsin inhibitor activities were abolished within 5 min. At these conditions urease was still active. Due to its high protein content, lack of soytoxin, and low levels of trypsin inhibitor, lectin, and urease it is suggested that Rio Balsas could be an alternative for breeding programmes aimed to improve the nutritional quality of soybeans.