Malachite green, an effective antifungal therapeutant used in fish culture, is a known teratogen. We developed a method to simultaneously detect both the chromatic and leuco forms of malachite green residues in the eggs, fry, and adult muscle tissue of rainbow trout (Oncorhynchus mykiss). Homogenates of these tissues were fortified with [14C] malachite green chloride and extracted with 1% (v/v) acetic acid in acetonitrile or in methanol. The extracts were partitioned with chloroform, dried, redissolved in mobile phase, and analyzed by liquid chromatography (LC) with postcolumn oxidation of leuco malachite green to the chromatic form. LC fractions were collected every 30 s for quantitation by scintillation counting. Recoveries of total [14C] malachite green chloride residue were 85 and 98% in eggs fortified with labeled malachite green at concentrations of 0.5 and 1.00 microgram/g, respectively; 68% in fry similarly fortified at a concentration of 0.65 microgram/g; and 66% in muscle homogenate similarly fortified at a level of 1.00 microgram/g. The method was tested under operational conditions by exposing adult rainbow trout to 1.00 mg/L [14C] malachite green chloride bath for 1 h. Muscle samples analyzed by sample oxidation and scintillation counting contained 1.3 and 0.5 microgram/g total malachite green chloride residues immediately after exposure and after a 5-day withdrawal period, respectively.