Effects of ethephon, AgNO3, and AVG on formation of embryogenic callus and somatic embryogenesis were examined in cultures of explants of spinach (Spinacia oleracea L. `Nippon') roots. At 10 μm, ethephon more than doubled the frequency of formation of embryogenic callus as compared to that in cultures without ethephon. Silver nitrate at 10 mm and AVG at 1 μm each inhibited formation of embryogenic callus but neither reduced the growth rate of established callus. When ethephon was applied at 10 μm during embryogenesis, it completely inhibited embryo development. By contrast, AgNO3 at 10 μm markedly increased the number of embryos. Results suggest that ethylene might be essential as a plant hormone for formation of embryogenic callus but inhibits somatic embryogenesis per se in cultures of explants of spinach roots. Chemical names used: 2-chloroethylphosphonic acid (ethephon), silver nitrate (AgNO3), aminoethoxyvinylglycine (AVG).