Chlorophyll–protein complexes of thylakoid membranes from rye plants (Secale cerealeL. cv. Puma) grown at warm and cold-hardening temperatures were investigated by gel electrophoresis. Complex IV from cold-grown tissue was detectable in the presence of dodecyl sulfate if and only if solubilization and electrophoresis were performed at 4 °C, whereas complex IV from warm-grown material was detectable if membrane solubilization and electrophoresis were performed at either 4 or 23 °C in the presence of dodecyl sulfate. In the presence of octyl-β-D-glucopyranoside, the chlorophyll–protein complexes from cold-grown tissue were less stable at 23 °C than those from warm-grown tissue. Regardless of the detergent used, there was always more oligomer of the light-harvesting complex present in samples prepared from thylakoid membranes of warm-grown tissue than those from membranes of cold-grown tissue. It is concluded that the pigment–protein interaction in those complexes associated with photosystem II and the light-harvesting chlorophylla/b– protein complex has been altered upon growth and development at cold-hardening temperatures.