We report here on the usefulness of the 14C-serotonin release assay for the laboratory confirmation of the clinical diagnosis of heparin-induced thrombocytopenia syndrome (HITS). Over the past 3 yrs, some 140 individual serum samples have been tested in our laboratory for heparin-associated anti-platelet activity ('heparin antibodies'). These included sera from 54 selected (4 positive, 50 negative) controls and a group of 86 patients where the test was requested on clinical grounds. Of 20 patients derived from within our institution, 7 out of 8 patients (88%) with good clinical probability of HITS were confirmed to have heparin platelet antibodies by the serotonin release assay. In contrast, only 2 out of 9 patients (22%) with a low clinical probability of HITS were shown to be positive by this procedure, as were 2 out of 3 patients (66%) deemed to have an 'intermediate' clinical probability of HITS. In addition, screening of 50 serum samples forming a 'negative-control non-HITS' group (either patients on heparin therapy without thrombocytopenia, patients with non-heparin associated thrombocytopenia [eg. ITP*, other drug related], or normal laboratory volunteers), consistently failed to display heparin associated anti-platelet activity by the 14C-serotonin release assay. In addition to the good specificity and sensitivity described above, the 14C-serotonin release assay was found to be nearly twice as sensitive when compared to the platelet aggregation procedure, and it is therefore a useful diagnostic test for the confirmation of clinically suspected HITS.