1. The mandibular organ of Macrobrachium rosenbergii was identified and cultured in the presence of [methyl-3H]-methionine. 2. Radiolabelled methyl farnesoate was extracted and quantitated from cultured glands and culture media of both males and females: variation in synthesis rates was observed (1558-52,652 DPM/hr per gland) between individual prawns. 3. No significant incorporation of isotope into hexane-extractable material was observed in any other tissue cultured. 4. The concentration of methyl farnesoate in the hemolymph of M. rosenbergii was determined using normal phase HPLC with cis-trans (non-biological isomer) as an internal standard. 5. Hemolymph from males contained 17.3 +/- 9.5 ng/ml methyl farnesoate. 6. Female hemolymph contained 24 +/- 13.1 ng/ml methyl farnesoate.