Chloroplast lamella preparations from spinach leaves were solubilized by a nonionic detergent, Nonidet P-40. Examination of turbidity, chlorophyll absorption spectra and the Hill reaction activity (dichlorophenol-indophenol photoreduction) revealed that the process of solubilization could be divided into four distinctive phases: Step I (at N/C ratios—Nonidet-chlorophyll ratio on molar basis—less than 4), II (N/C) from 4 to 11), III (N/C from 11 to 60) and IV (N/C beyond 60). The turbidily decreased at Steps II and III with a platean at N/C II; the Hill reaction activity was lost at Step II, while the physical state of chlorophyll molecules as judged by the shift of absorption maximum changed in Steps III and IV. Circumstantial evidence indicated the separation of lipids from proteins mainly at Step IV. The preparation, obtained in a similar way but from chloroplasts washed with water and 1 mM EDTA and purified by freezing, showed no Hill reaction and had only an insignificant amount of the components responsible for the turbidity changes in Step II. The results strongly suggest the lipoprotein subunit nature of the lamella membrane in which at least two types, intra- and intersubunit, hydrophobic association are present. The usefulness of the stepwise detergent treatment technique in membrane biochemistry is suggested.