SUMMARY

This investigation has been focused on the significance of residual growth in the latent period after ultraviolet irradiation of a lysogenic strain of Staphylococcus aureus (I 1 I). It was shown that the organisms divide once or twice before the development of free phage and the number of infective centres are increased by about IOO % in 30 min. after irradiation. When celbenin (sodium 6-(2,6 dimethoxy benzamido) penicillinate) was added to the culture after irradiation, the number of infective centres de-creased progressively, and no free phage developed. A delay of IO, 20 or 3o min. in introduction ofthe drug allowed an increasing fraction of the infective centres to develop and to yield free phage. These observations lead to the suggestion that ultraviolet irradiation inhibits the synthesis of new repressor of vegetative phage development, but has no influence on repressor molecules present beforehand. Residual growth of the irradiated bacteria serves to dilute the intracellular repressor con-centration below an effective level