Oral fluid (OF) is a complex liquid secretion, which can be collected painlessly, rapidly and non-invasively by low trained persons. The sample collection could be supervised without disturbing objects providing it. Recently OF has attracted clinical and forensic toxicologists attention due to its application as an alternative biological specimen of the blood that can provide an estimation of exposure to drugs of abuse. The present research is aimed to the characterization of OF specimen in order to determine suitable parameters as markers for its identity/dilution. Further study on the effects of storage temperature, additives presence and storage period on sample stability was also performed. The data obtained revealed that the protein profile and the α-amylase activity can be used as markers of OF samples identity, while creatinine levels could serve as an indicator for dilution/stability of oral fluid. The experimental results showed that creatinine concentration depends on storage conditions. The sample storage at 4°C for a month leads to increasing of creatinine levels up to 50%; the process is partially inhibited by the presence of Na₂EDTA or NaN₃ and the amount of creatinine increases up to 10% as compared to the initial value. The storage of samples at −20°C for a month leads to significant decrease of initial creatinine concentration even in the presence of stabilizer/preservative.