The distribution of small labeled latex test particles has been studied after intravenous administration to the rabbit using the noninvasive technique of gamma scintigraphy. The reticuloendothelial system was suppressed using polystyrene latex particles and dextran sulfate as blocking agents. Both materials gave rise to significant uptake of particles in the lungs of the order of 60% of the administered dose. The effect seen with suppression with latex particles remained unaltered over an 8-d period, while for suppression with dextran sulfate the effect was transient, and particles that were initially deposited in the lungs were subsequently redistributed to the liver. The results are discussed in terms of previous studies on reticuloendothelial suppression using different agents, test colloids, and animal models.