Erythrocyte sedimentation curves are analyzed by determining the constants of a linear relation between logarithm of sedimentation distance and logarithm of time that obtains for the initial part of the sedimentation process. The sedimentation curves obtained from dogs exposed to x-rays, or injected with radiostrontium or plutonium were analyzed in this manner. The pathologic sedimentation curves fall into two groups. Dogs that received sublethal dosages of x-ray, radiostrontium or plutonium showed a response (phase I response) in which decreased slope of the log-log line is associated with increased intercept. Dogs that received dosages in or beyond the LD50 range showed, during the period of maximum acute death rate, a further increase in intercept associated with an increase in slope (phase II response). An increased slope indicates an increased rate of rouleau growth. It is inferred that the phase I response is due to decreased hematocrit and the phase II response to an increased titer of an agglutinating principle.