A two‐dimensional, highly reproducible, analytical separation procedure for Triton/urea extractable proteins, using isoelectric focusing in the first dimension and gelelectrophoresis with sodium dodecylsulfate (SDS) in the second, is described. The separation of erythrocyte membrane proteins compares the reliability with that of published procedures. From the thylakoid membranes of the green alga Chlamydomonas reinhardii and of spinach, two‐dimensional maps of the Triton/urea extractable proteins were established, containing 58 and 33 peptide spots, respectively, which were characterized by their molecular weights and apparent isoelectric points. In Chlamydomonas some of the spots could be attributed to the photosystem I and II particles. A few major spots in the maps of the thylakoid proteins from the two non‐related plants were found at the same relative positions. A one‐dimensional electrophoretic comparison of the SDS and the Triton/urea‐extractable proteins showed that in the Triton/urea extract of thylakoid membranes some proteins of the photosystem II region, and in that of erythrocyte membranes, some components of the band 3 and 5 region are reduced or missing.

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