Abstract The median incubation period (IP 50) of sowthistle yellow vein virus (SYVV) at 25 ° increased approximately 24 hours for every 10-fold dilution of plant-derived inoculum injected into late-instar larvae of Hyperomyzus lactucae. The percentage of aphids infected and the duration of the incubation period were negatively correlated. Thus either the IP 50 or the probability of infecting an aphid could, within limits, be used to estimate the relative amount of active virus in the inoculum. Particle counts with the electron microscope showed that an injected dose that gave a 7-day IP 50 had about 10 5 particles. A graphic analysis of the data indicated that the minimum dose that could be resolved by this system of bioassay would contain about 10 2.5 particles.