Influence of imidazole carboxamide (DTIC, NSC-45388)on some platelet functions, plasma coagulation, and fibrinolysisin vitrowas investigated, using different concentrations of the drug (80 and 160 μg/ml). The lower concentration, which corresponds to that obtained in plasma after administration of therapeutic doses in man, did not significantly influence any of the important platelet functions. It is concluded that after current dosage of the drug, bleeding disorders caused by impairment of platelet functions are improbable. However, following the higher concentration of DTIC, a significant inhibition of the increase in light transmission of platelet-rich plasma provoked by standardized freezing and thawing procedure, was demonstrated. Furthermore, the same dose of the drug caused partial retardation of the retraction of reptilase clots. The authors assume that DTIC has a “stabilizing effect” on the cell membrane, particularly in higher concentrations.