The morphology and growth rates of cell lines from the mosquitoes Aedes aegypti (20A), Anopheles stephensi (43), Anopheles gambiae (55) , Aedes malayensis (60), Aedes pseudoscutellaris (61), and from the toad Xenopus laevis (XTC-2) in various media were examined. Minimum infectious dose experiments with eight arboviruses demonstrated that the 60 and 61 cells were the most highly sensitive to infection. The 20A, 43 and 55 cells were comparatively less sensitive, although the XTC-2 cells were comparable in sensitivity to monkey kidney cells. Comparisons of the growth of 23 arboviruses in all the lines were made. The XTC-2 cells supported the growth of nearly all the viruses tested. Most of the viruses multiplied in the mosquito cells to varying degrees, but none of the tick-borne viruses tested grew in the mosquito cells. Cytopathic effects (CPE) and plaques were produced by a number of viruses in the XTC-2 cells, and this cell line is routinely used for the low temperature assay of a range of viruses. CPE in the mosquito cells was observed only in the 60 and 61 cells, and only with three flaviviruses. Attempts to produce plaques in all the mosquito cell lines under carboxymethylcellulose overlay were unsuccessful. Virus carrier cultures were established and alterations in the properties of progeny virus were examined over long periods. The results suggest the presence of an interferon-like system in the XTC-2 cells, but not in the mosquito cells. Comparison of the results obtained with published work shows a high degree of correlation. Aspects of virus-vector relationships in vitro are discussed, and some of the potential practical applications of these cell lines have been examined.