We previously demonstrated that human osteosarcoma cells (SAOS-2) induce contact-dependent apoptosis in endothelium, and expected similar apoptosis in human gingival fibroblasts (h-GF) using SAOS-2 alkaline phosphatase (AP) to identify cells. However, h-GF apoptosis did not occur, despite reduction in AP-negative h-GF number (p < 0.01) and enhancement of this by h-GF TNFa pretreatment (p < 0.01). We suggest that TNFa- enhanced transfer of membrane AP from SAOS-2 to h-GF would explain these data. This idea was investigated using fluorescence prelabelled cells and confocal laser scanning microscopy. Co-cultures of membrane-labelled h-GF (marker-DiO) and SAOS-2 (marker-DiD) generated dual-labelled cells, primarily at the expense of single labelled h- GF (p < 0.001), suggesting predominant membrane transfer from SAOS-2 to h-GF. However, opposite directional transfer predominated when membrane labels were reversed; SAOS-2 further expressed green fluorescent protein (GFP) in cytoplasm and nuclei, and h-GF additionally bore nuclear label (Syto59) (p < 0.001). Cytoplasmic exchange was investigated using h-GF prelabelled with cytoplasmic DDAO-SE and nuclear Syto59, co-cultured with SAOS-2 expressing GFP in cytoplasm and nuclei, and predominant cytoplasmic marker transferred from h-GF to SAOS-2 (p < 0.05). Pretreating h-GF with TNFa increased exchange of membrane markers (p < 0.04) but did not affect either cell surface area profile or circularity. Dual-labelled cells had a morphological phenotype differing from SAOS-2 and h-GF (p < 0.001). Time-lapse microscopy revealed extensive migration of SAOS-2 and cell process contact with h-GF, with the appearance of SAOS-2 indulging in 'cellular sipping' from h-GF. Similar exchange of membrane was seen between h-GF and with other cell lines (melanoma MeIRMu, NM39, WMM175, MM200-B12; osteosarcoma U20S; ovarian carcinoma cells PE01, PE04 and COLO316), while cytoplasmic sharing was also seen in all cell lines other than U20S. We suggest that in some neoplasms, cellular sipping may contribute to phenotypic change and the generation of diverse tumour cell populations independent of genetic change, raising the possibility of a role in tumour progression.