Postmortem biochemistry may provide significant information in determining the cause of death. Due to the rapid postmortem breakdown of metabolism and active membrane transport only analytes which are stable in blood can be determined on this fluid compartment, other parameters have to be analysed on other fluid compartments like vitreous humor (VH). However, using another fluid compartment as a mirror of blood at the moment of death involves severe methodical problems. The conceptual problems of reference values in vitreous humor as a mirror of blood are addressed. Additionally, the necessary steps to be taken before calculating the discriminating values between "normal" and "diseased" are described. For all chemical determinations, a clear definition of the site of sample acquisition is necessary. Up to now chemical determinations on alternative fluids have mainly been carried out using instruments calibrated for urine or serum. Developing calibrated methods for analysis of alternative fluids is a further task for the future.