Objective Research and provision of data on macrophages by cytological synovial fluid analysis and light microscopy in horses with septic arthritis Material and methods Records of 167 synovial fluid samples were evaluated and subdivided into different groups: (1) non-septic, (2) haematogenous septic arthritis in foals and (3) traumatic/iatrogenic septic arthritis. The effect of joint lavage on synovial fluid cytology and on the occurrence of macrophage phenotypes was investigated. Results Regardless of aetiology and age of the horse, macrophage concentrations in synovial sepsis are decreased to a median of 5–6 % (unaffected joints: 23.5 %) and further diminished by joint lavage. Microscopic assessment led to the identification of 4 phenotypes. Morphological characteristics of type 1 showed similarities to monocytes and predominated in unaffected and in septic joints after lavage. Conclusion and clinical relevance Macrophages are highly versatile by altering their phenotype. A morphological assessment by light microscopy is easily applicable. Type 1 presumably contributes to joint homeostasis.