In Swiss mice inoculated intravenously with the Friend mouse leukemia virus, the mean log spleen weight at an arbitrary time after infection was a function of log virus dose. The dose-response function was S-shaped, with an upper shoulder or a plateau at high doses of virus, a linear phase, and a lower plateau with low doses of virus. Fitting of regression lines to the linear phase and calculation of the virus dosage corresponding to spleen-weight response of 500 mg. permitted reproducible quantitation of the agent. This procedure permits reasonable estimates of virus titer to be made on the basis of spleen-weight response to a single dilution of virus.