Embryoid bodies (EB) derived from mouse embryonic stem cells were used to establish a new in vitro model for cellular defence processes. Therefore, the differentiation of endothelial cells (analyzed by PECAM-1 and VE-cadherin) and leukocytes (detected by CD45) in the EB, especially neutrophils (CD16- positive or expression of neutrophil antigen) and macrophages (identified by the expression of CD68 and CD14) were analyzed in a time-dependent manner. To simulate inflammation, EBs were treated with lipopolysaccharide (LPS) originating from E. coli K-235. A concentration-dependent increase in CD14-expression was observed. Isolated leukocytes responded to 10 mM LPS with Ca2+ signals. Furthermore we used confrontation cultures of EBs with multicellular tumour spheroids (MTS), which were generated from the 4T1 mouse mammary carcinoma cell line. During tumour-induced angiogenesis we observed an invasion of leukocytes into the MTS. Following 4 days of co-culture CD45+ leukocytes originating from the EB were identified all over the confrontation culture, whereas CD68+ macrophages were mainly located at the periphery of the MTS. Our findings indicate that embryonic stem cell-derived EBs provide a suitable in vitro model for cellular defence.