Splenocytes were collected from WT and heterozygous B-hCSF1 (H/+) mice stimulated with LPS in vivo, and analyzed by ELISA with species-specific anti-CSF1 ELISA kit. Mouse CSF1 was detectable in WT and heterozygous B-hCSF1 mice. Human CSF1 was exclusively detectable in heterozygous B-hCSF1 but not WT mice.