Splenic dynamics of 111In-labeled platelets and platelet-associated IgG in 33 patients with idiopathic thrombocytopenic purpura (ITP) were studied. Two half-lives were calculated for the biexponential splenic time-activity curve after i.v. injection of 111In-labeled platelets. There was no difference in the mean half-life of the rapid component of the splenic curve (ST1) when patients with negative or slightly positive platelet suspension immunofluorescence test (PSIFT) were compared to those with strongly positive PSIFT (3.0 +/- 0.7 min vs. 3.6 +/- 0.4, p greater than 0.05). Mean half-life of the slow component of the splenic curve (ST2) was found to be longer in patients with a strongly positive than a negative or weakly positive PSIFT (26 +/- 5 min vs. 13.2 +/- 1.0 min, p less than 0.01). It seems that determination of the two components of the splenic time-activity curve provides a useful method for studying platelet kinetics in ITP.