- PURPOSE: To determine bystander and direct effects of ionizing radiation on eight fish cell lines. MATERIALS AND METHODS: Fish cell lines were irradiated at a range of doses from 0.5 - 5 Gy. The Irradiated Cell Conditioned Medium (ICCM) was then harvested and placed onto a HPV-G, reporter cell line as well as onto autologous fish cell lines. Cloning efficiency (CE) was the end point used. The HPV-G reporter cell line was chosen because this cell line is capable of transmitting and producing the bystander effect. RESULTS: Four of the eight fish cell lines were clonogenic. These, with the exception of RTG-2 cells, showed increased CE when ICCM was tested on unirradiated autologous cells or on HPV-G cells. ICCM from RTG-2 cells reduced survival. The non-clonogenic cells ICCM tested on HPV-G all showed increased CE. CONCLUSIONS: The results show that both bystander signal production and cellular response varies depending on the cell line and that in general signals from established fish cells do not produce death inducing bystander effects. Thus, the comparison of the effect from fish cell ICCM on autologous cells or HPV-G human cells allowed us to separate signal production from response. In almost all cases, for both non-clonogenic and clonogenic fish cell lines, the HPV-G recipient cell line showed an increase in percent survival compared to controls while the clonogenic fish cell lines do not appear to respond.