The in vitro model of tumour infiltrating macrophages (TIM)-tumour interactions in which monocytes and monocyte-derived macrophages (MDM) are cultured with cancer cells was used to assess immunophenotypic changes of interacting cells. Following short cocultures, monocytes, MDM and tumour cells were sorted out by FACS and the expression of several determinants was evaluated. Monocytes showed the induction of CD44v6 and v7/8, and up-regulation of CD16 (Fc gamma RIII), CD54 (ICAM-1), CD68 (macrophage maturation marker) and CD86 (costimulatory molecule B7.2). The increased expression of CD11a (LFA-1) and CD58 (LFA-3) was noted on some cancer cells. Up-regulation of TNFRII and HLA-DR was observed on both types of cells. MDM shared similar changes. Contact of monocytes, but not of MDM, with tumour cells led to Fas-FasL-dependent apoptosis of both types of cells. This study suggests that the immunophenotype of monocytes/macrophages and cancer cells may be modified during their bidirectional interactions in the absence of other microenvironmental elements that are present in the tumour stroma.