In view of the encouraging results obtained in active mouse protection experiments, several batches of live oral vaccine prepared from streptomycin-dependent Shigella flexneri were tested in vitro for any indication of reversion to streptomycin-independence and in man for their reactogenicity.The in vitro tests provided no evidence of reversion.In man, major reactions were observed in only 1.4% of volunteers, all those so affected being heterotypic Shigella carriers in whom clinical exacerbation was induced. Both the intensity and the frequency of the minor reactions observed in other volunteers depended on the size of the vaccine dose administered. The dosage was also directly related to the duration of shedding of the vaccinal strain in the faeces and to the clinical exacerbation shown by heterotypic carriers.Both the phagocytic activity and the serum antibody titres were equally low before and after vaccination, but post-vaccination copro-antibody titres were 8-32 times higher with homologous than with heterologous antigens.