Rodent cells are used widely to manufacture recombinant proteins for pharmaceutical use in humans and animals. However, all rodent cell lines express endogenous retroviruses that require appropriate testing regimes for identification and characterisation. In this communication we report the results of transmission electron microscopy, reverse transcriptase assay and infectious virus assays for retrovirus in 185 manufacturer cell banks of mouse, rat or hamster origin. The results indicated considerable variability of retroviral expression levels by transmission electron microscopy and reverse transcriptase assay, but nevertheless characteristic features of each cell type were observed. Infectious retrovirus was detected in mouse myeloma and hybridoma cell lines, but not in cell lines of hamster or rat origin. There was no evidence of contamination of cell banks with exogenous retrovirus. The results of retroviral characterisation of the parental mouse cell lines NS0, NS-1 and Sp2/0Ag14 by the above assays were consistent with the results of the survey. Co-cultivation of the above parental mouse cell lines with mouse and human cell lines suggested that the ability to infect human cells was related to threshold susceptibility of cell types and the levels of expression of infectious xenotropic retrovirus by mouse cells.