The ability of 10 different antigens to stimulate uptake of 125I UDR, as a measure of DNA synthesis in T cells,1 was investigated. There was a good correlation between the stimulatory capacity of the antigens and their antigenicity. Five of the antigens studied have been reported to be “thymus-independent.”. Three of them (brucella, POL and ferritin) were among the best T cell stimulators while the other two (pneumococcal polysaccharide and PVP) were the poorest. This discrepancy suggests that the basis of their reported “thymus-independency” is different. An explanation for the difference is offered.