This study was designed to evaluate the effects of varying concentrations of platelets, white blood cells (WBC) and Fab fragments of a monoclonal antibody (c7E3, Reopro) directed at the platelet GpIIb-IIIa receptor complex on ACT-based clot ratio values (hemoSTATUS assay) in healthy volunteers. These measurements were made in heparinized whole blood from 10 normal volunteers in which either platelet or WBC concentrations had been varied by differential centrifugation. In addition, blood collected in either heparin or argatroban was incubated with varying concentrations of c7E3 (Reopro). Clot ratio values (%Maximal) in normal blood did not decrease until average platelet counts were less than 50,000. A marked reduction in clot ratios was observed when WBC concentration increased above or decreased below baseline clot ratios within each patient. Strong linear relationships were observed between white cell concentration and clot ratio values when white cell concentrations were either less or greater than baseline values. When argatroban was used as an anticoagulant, inverse relationships were demonstrated between clot ratio values and increasing c7E3 concentration (Ch 3: r = -0.33, Ch4: r = -0.84, Ch5: r = -0.87, Ch 6: r = -0.71). ACT-based clot ratio values determined in heparinized whole blood presumably reflecting PAF inducible platelet procoagulant activity, are affected by platelet concentration when counts are less than 50,000/microliter. The hemoSTATUS test was also found to be affected by WBC concentration since clot ratio values decreased when WBC counts were below 4,000/microliter or above 9,000/microliter. A dose-dependent reduction in clot ratio values was also observed with increasing concentrations of c7E3. This test can reliably detect platelet dysfunction only if the platelet count is > 50,000 and the WBC is normal.