Major depressive disorder (MDD) is a debilitating disease worldwide and occurs with a high prevalence in elderly individuals. In attempt to search for herbal medicine with antidepressant effect, each in vitro model must be properly validated. The objective of this study was to determine the effect of Senna siamea Lam. (Cassod tree) leaf extracts prepared using ethanol and water on activity of DAT, NET, SERT and total MATs in in vitro model, human neuroblastoma LAN-5 cell line using a commercial neurotransmitter transporter reuptake assay kit. After measuring the intracellular fluorescence, we calculated RFU area below curve reflecting the activity of dopamine transporter (DAT), norepinephrine transporter (NET), serotonin transporter (SERT) and all monoamine neurotransmitter transporters (MATs) in LAN-5 cells after treatment for 30 min with 25 and 100 μg/mL of Senna siamea Lam. leaf extracts in the absence or presence of combining antidepressants. We observed the fluorescence intensity below zero or minus values, thus reflecting assay interference. The inference was stronger as reflected by lesser intracellular fluorescence intensity when increasing concentrations of Senna siamea Lam. extracts. Ethanol extract resulted in a stronger interference as compared to that of water extract. To confirm that Senna siamea Lam. leaf extracts containing certain fluorescent compounds with excitation 440 nm and emission 520 nm, we measured the fluorescence of Senna siamea extracts compared with reagent blank and Mentha cordifolia Opiz ex. Frezen (Kitchen mint) leaf and Centella asiatica L (Urm.) (Gotu kola) whole plant extracts as control herbal extracts using spectrofluorometer with the excitation wavelength of 440 nm and emission wavelengths of 480-600 nm for scanning, and we found the concentration-dependent and solvent type-dependent manner in agreement with the observation in neurotransmitter transporter reuptake assay. Therefore, Senna siamea leaf extracts possessed fluorescence characteristics possibly due to certain fluorescent compounds and could be interfered in a fluorescence-based assay. It is highly recommended to know the chemical nature of each plant extract before selecting any biological assay.