111In-labelled platelets were used for analysing platelet dynamics in 43 patients with idiopathic thrombocytopenic purpura (ITP). The detected time-activity curves, recorded with a gamma camera, were analysed by three methods: two- and three-compartment models, and an open model in which only the splenic curve was analysed. In the two-compartment model the mean rate constant from blood to spleen was 0.328 +/- 0.028 min-1 (mean +/- SEM) and from spleen to blood 0.061 +/- 0.007 min-1, whereas in the three-compartment model the corresponding values were 0.236 +/- 0.020 and 0.044 +/- 0.007 min-1, respectively. The mean rate constant from blood to liver was 0.466 +/- 0.149 min-1 and from liver to blood 0.341 +/- 0.106 min-1 as derived from the three-compartment model. The rate constant from spleen to blood, as determined from the three-compartment model, was significantly higher in patients with a strongly positive result for platelet-associated auto-antibodies (platelet suspension immunofluorescence test (PSIFT] than in patients with a negative PSIFT. The mean hepatic net rate in patients with a high level of antibodies is into the liver, while in patients with little or no antibodies the net rate is into the blood pool. The mean half-life for the fast component of the inverted splenic curve was 2.5 +/- 0.2 min and for the slow component 16 +/- 2 min. In patients with a strongly positive PSIFT the half-life for the slow component was significantly longer than in patients with a negative PSIFT.(ABSTRACT TRUNCATED AT 250 WORDS)