Twenty African arboviruses, five alphaviruses, nine flaviviruses, three Bunyamwera Group viruses, two Bwamba Group viruses and one ungrouped virus were titrated in parallel in 11 cell systems in suckling mice and adult mice. The relative sensitivities of the in vitro and in vivo systems have been compared. The highest infectivities were obtained in suckling mice. Vero and LLC-MK2 cells produced plaques with the greatest number of viruses and Semliki Forest virus grew most readily. Ntaya virus and Dengue 1 virus were difficult to culture in vitro and Zika virus yielded better in cell culture than in adult or suckling mice. In vitro and in vivo neutralization tests were made on human sera in groups of 50. Each group of sera was tested against one of five viruses, representative of three of the arbovirus groups titrated. Good agreement was obtained between the two test systems with West Nile, O'nyong-nyong and Wesselsbron viruses but there were significant differences in results obtained with Germiston and Pongola viruses.