The discovery of aerobes and anaerobes by cultivation of bacteria on agar plates with and without oxygen was a milestone in the classification and therapy of bacterial infections. However, doubling time for microorganisms can be as short as several minutes, thus cultivation methods are not applicable to the relatively slowly dividing (mammalian) cells. A plethora of clinical trials shows that the presence of anaerobic tumor cells within a tumor correlates with a worse clinical outcome. These cells are radio- and chemotherapy resistant and often display a stem-cell like aggressive phenotype (3, 4). Solid and hematological malignancies contain tumor cells that can grow in an anoxic (severely oxygen deficient) micro-environment (5-7). It is also well known that the role of immune cells in inflammation depends on their capability to maintain function (cytokine production and secretion) and growth under severe oxygen deficiency. Anoxia improves neutrophil viability and bacterial clearance by neutrophils and anoxic conditioning might increase the efficacy of immune cell transfusion in humans (8). Cells growing under anoxia produce energy and building blocks for macromolecules by anaerobic glycolysis – thus independent from mitochondrial (oxygen dependent) respiration (9). Employment of LDH and PK M2 under anoxia (compared to normoxia) indicates fast cellular growth under anoxia (2, 10) and allows faster incorporation of glucose metabolites into biomass facilitating cell growth without oxygen (10, 11) (Fig. 2). Measuring these enzyme activities with this tool allows rapid and easy detection of fast growing aerotolerant anaerobic mammalian cells in solid tissue, body fluids or cell culture.