Comparative studies on isolated chromatophores and on sectioned cells of the photosynthetic bacterium Rhodospirillum rubrum confirm the assumption expressed in earlier investigations that the photochemically active chromatophores isolated from disrupted cells represent structural chlorophyll-bearing components of the protoplast.

Actively growing cells from light-grown cultures about 12 hours old do not release chromatophores when disrupted in dilute buffers, but do release smaller, chlorophyll-containing structures about 25 mµ in diameter. Sections of such cells do not reveal chromatophores, but contain in the ground cytoplasm numerous particles somewhat smaller in size than the 25 mµ chlorophyll-containing particles released from disrupted cells. Similar particles are obtained by the sonication of isolated chromatophores obtained from cells of 1-day-old cultures.

The small, subchromatophore particles described here appear to be functionally complete units which are photochemically active in photo-oxidation, photoreduction, and photophosphorylation, and it is postulated that they represent the basic biochemical and structural components of the chromatophore.