CI < 1, = 1, and > 1 indicates synergism, additivity and antagonism, respectively. More precisely, symbols +, ++ and +++ describe slight synergism (CI between 0.85 and 0.90), moderate synergism (CI between 0.7 and 0.85) and synergism (CI between 0.3 and 0.7), respectively. Symbols -, --, --- and ---- indicate slight antagonism (CI between 1.10 and 1.20), moderate antagonism (CI between 1.20 and 1.45), antagonism (CI between 1.45 and 3.3) and strong antagonism (CI > 3.3), respectively. Nearly additive effects (CI in the range 0.90 and 1.10) are represented by the symbol ±. ED50, ED75 and ED90 indicate 50%, 75% and 90% inhibition of growth, respectively. Equipotent amounts of the two agents were combined. In most experiments employing X-rays, various ratios were tested. Each CI determination resulted from at least three independent experiments evaluating at least five different doses or dilutions of the two agents.