The time required for drosophilas to become CO 2 sensitive (incubation time) following injection with a suspension of genoïdes, has been measured and correlated with the number of infectious units introduced into the fly. This number is derived from the percentage of flies which remain resistant following injection with a sufficiently diluted suspension. With moderately concentrated suspensions, a linear relation is observed between incubation time and the logarithm of infectious units. However, when the mean concentration falls to less than one unit per injected fly, the incubation time remains at a fixed maximum value. For high concentrations of genoïdes, the semilogarithmic linear relation does not hold, its slope becoming less steep. A new method for assaying the titer of a suspension of genoïdes is derived from the established relation.