SummaryCurrently, environmental enrichment is a very common means of improving animal wellbeing, especially for laboratory animals. Although environmental enrichment seems to be a possible way for improving the well-being of anim als, the consideration of housing laboratory anim als should not only focus solely on animal well-being, manpower and economics but also on the precision and accuracy of the experimental results. T he purpose of the present study was to evaluate the effects of enriched cages (nest box, nesting material, clim bing bar) on body weight, haemat ological data and ®nal organ weights.BALB=c, C57BL=6 and A=J mice, originated from Harlan Winkelmann, were used for the experimentsÐ16 animals of each strain. Animals at 3 weeks of age were marked and separated random ly to enriched or non-enriched cages, in groups of four, half for each housing condition. Both cages were type III Makrolon cages, only the enriched cages contained a nest box, a wood bar for clim bing and nesting material. Animals were kept in a clean anim al room under speci®c pathogen free (SPF) conditions. Body weights were recorded every week. Blood samples were collected at 14 weeks of age (white blood cells (WBC ), red blood cells (RBC ), haemoglobin (HGB), and haematocrit (HCT ) were analysed). At 15 weeks of age, the animals were euthanized by CO 2 in their home cages, and ®nal body weight and organ weights (heart, liver, kidney, adrenal, spleen and uterus) were recorded im mediately.Although nearly all the test variables were not affec ted by environmental enrichment in their mean values, the enriched group showed higher coef®cients of variat ion in many variables, and strain differences of both housing conditions were not consistent. T he in¯uences of enrichment were shown to be strain-and test-dependent. Such effects may lead to an increase in the number of animals which is necessary or may change the experimental results, especially when a study, using enriched housing conditions, focuses on strain differences.Since the same enrichment design can result in different in¯uences, a positive or a negative or no adverse effect, due to the strain and the variables studied, researchers need to collect more information before enrichment designs are introduced into experimental plans.Keywords Inbred mice; strain differences; environmental enrichment; haematologic al analysis; organ weightTo assure a high precision of results in animal experiments, high levels of standardization have been introduced into laboratory animal science over many years, especially in respect to genetics, hygiene and environment, including feeding and housing. T his high level of standardization has been largely