1. At equivalent concentrations, cobalt was more effective than manganese in inducing chlorosis and necrosis in tomato plants. 2. Chlorotic plants suffering from either cobalt or manganese toxicity alone, or from the two conditions together, contained as much or more iron than normal plants, and chlorotic leaves responded positively to painting with a solution of ferrous sulphate. 3. When cultures were supplied with 5 p.p.m. Mn, distinct injury of the lower leaves occurred regardless of the cobalt supply, but the addition of cobalt up to 0·01 p.p.m. brought about a gradual decrease in the chlorosis of the top leaves and an increase in dry weight. 4. With cobalt at the 0·1 p.p.m. level, severe chlorosis and necrosis occurred and growth was stunted, but by raising the manganese level to 5·0 p.p.m. a substantial antidoting effect on the toxicity was obtained. 5. Cobalt and manganese had an antidoting effect on each other, whereby at high manganese supplies small quantities of cobalt alleviated manganese toxicity, while toxic concentrations of cobalt were antidoted by high levels of manganese.