CSF-1 (colony stimulating factor-1), initially considered to be a monocyte specific growth and differentiation factor [4], has recently been shown to be produced in human endometrium [16], placenta [7], as well as in numerous solid tumors [19-23, 26, 27]. The CSF-1 receptor (a protein product of c-fms) [24] is a member of the tyrosine kinase receptor family and an autocrine or paracrine mechanism of activation has been suggested. Overactivation of this receptor can lead to a malignant phenotype in various cell systems [20, 21]. We review the biology of CSF-1 and fms expression in normal as well as in malignant tissues with particular reference to a potential role for CSF-1 in breast tumour invasion.