The present study was designed to investigate the survival of platelets of differing densities in normal and post-splenectomized subjects. Autologous platelets, labelled with 111In-oxine, were reinjected into normal subjects (n = 12); 63% were recovered in the circulation and their survival curve was linear with a T 1/2 of 4.5 d. When the platelets were layered onto a continuous Percoll gradient, they formed a band extending between 1.040 and 1.080 g ml-1. After fractionation of the gradient the specific radioactivity of 111In platelets recovered was measured. The specific activity of low density platelets (average 1.050 g ml-1) decreased rapidly with a T 1/2 of 2.0 d, whilst medium density platelets (average 1.060 g ml-1) survived with a T 1/2 of 4.5 d; high density platelets (average 1.073 g ml-1) exhibited a T 1/2 greater than 5.0 d. This latter population of high density platelets also showed a significant increase in specific activity on the first day following injection. In post-splenectomy subjects a similar relationship between density and 111In associated activity was observed but no increase in the specific activity of the dense platelets on day 1 was observed. We conclude that high density autologous 111In-platelets are preferentially retained in the spleen and have a more prolonged survival than those of lower density.