The use of mice as animal models in biomedical research allows the standardization of genetic background, housing conditions as well as experimental protocols, which all affect phenotypic variability. The phenotypic variability within the experimental unit determines the choice of the group size which is necessary for achieving valid and reproducible results. In this study, the variability of clinical chemical and hematological parameters which represent a comprehensive blood screen of laboratory mice, as well as of immunological parameters and behavioral tests was analyzed in data sets which have been submitted to the Mouse Phenome Database for mouse strains which are predominantly used in biomedical research. Most of the clinical chemical and hematological parameters–except of some parameters being known for their high variability–showed an average coefficient of variation (CV = standard deviation / mean) below 0.25. Most immunological parameters measured in blood samples had a CV between 0.2 and 0.4. The behavioral tests showed a CV between 0.4 and 0.6, or higher. In addition, a large range of the CV was found for most parameters/tests between and within the selected projects. This clearly demonstrates the appearance of unpredictable major interactions between genotype, environment and experiment regarding the variability of the parameters and tests analyzed.