Plasma and thylakoid membranes were isolated and purified from the cyanobacterium Anacystis nidulans. Spectrophotometric examination of acetone extracts gave major absorption bands resulting from carotenoids and chlorophyll a in plasma and thylakoid membranes, respectively. Only a very small absorption peak at 663 nm was detected in acetone extracts of plasma membranes which, in contrast to the corresponding peak from thylakoid membranes, could not be extracted into n-hexane; methanol, on the other hand, was effective with both plasma and thylakoid membranes. Aqueous membrane suspensions excited at 435 nm gave strong fluorescence emission at 662 nm for plasma membranes, but only a very small one for thylakoid membranes which had been adjusted to equal absorbance at 678 nm. Excitation spectra of the 668 nm fluorescence emission peak in acetone extracts of plasma and thylakoid membranes were strikingly different from each other. Finally, high performance liquid chromatography afforded clear-cut preparative separation of the two "chlorophyll-like" pigments in plasma and thylakoid membranes, respectively, and identification by comparison with retention characteristics known from the literature, together with a pure chlorophyll a standard. Our results indicate that the highly fluorescent and polar "chlorophyll-like" pigment in plasma membranes of Anacystis is a chlorophyll precursor, viz. chlorophyllide a.