Electron microscopy of thin sections of 569 fixed under conditions that demonstrate 749c to have only one mesosome per bacterium has shown the former to possess consistently more than one. Treatment of outgrowing 569 spores with actinomycin-D results in the appearance of extra membrane-like material lying between the plasma membrane and the new wall. Actinomycin-D causes almost immediate cessation of exponential growth in 569 but extra membrane appears after a lag of approximately half a generation time. Autoradiography of thin sections of exponentially growing bacteria treated with actinomycin-D and labelled during inhibition with tritiated glycerol shows that the grain count is increased over areas of sections where extra membrane is present. Analysis of the lipids extracted shows that some of the labelled glycerol is incorporated during inhibition into material similar to or identical with the normal phospholipids. Virtually no incorporation of labelled amino acid occurred in trichloroacetic acid precipitates during actinomycin-D inhibition. These observations, which suggest that synthesis of new membrane occurs during actinomycin-D inhibition, are discussed in relation to a possible origin from mesosomes.


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