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The myorelaxant thiocolchicoside (TC), an analogue of colchicine (COL), was assayed in plasma and urine by a radioimmunoassay (RIA) using a cross-reacting COL-specific polyclonal antibody. Cross-reactivity was 56 for TC, giving a limit of quantification of 0.5 ng/ml and a linear response from 0.5 to 100 ng/ml. Specificity was checked by cross-reactivity studies with COL analogues and by using liquid chromatography and RIA in tandem on urine samples. Two immunoreactive peaks were detected, but the nonspecific peak represented 2 of the total urine concentration of TC. Pharmacokinetics of TC following infusion of 4 mg in two subjects revealed a moderate distribution (Vss from 31 to 35 L) and mainly extrarenal elimination (75 of the total body clearance). Terminal half-lives ranged from 2.4 to 2.7 h in plasma and from 3.2 to 3.7 h in urine.