The abilities of two races of Arabidopsis thaliana L. (Heyn), Landsberg erecta and Columbia, to cold harden were examined. Landsberg, grown at 22 to 24 degrees C, increased in freezing tolerance from an initial 50% lethal temperature (LT(50)) of about -3 degrees C to an LT(50) of about -6 degrees C after 24 hours at 4 degrees C; LT(50) values of -8 to -10 degrees C were achieved after 8 to 9 days at 4 degrees C. Similar increases in freezing tolerance were obtained with Columbia. In vitro translation of poly(A(+)) RNA isolated from control and cold-treated Columbia showed that low temperature induced changes in the population of translatable mRNAs. An mRNA encoding a polypeptide of about 160 kilodaltons (isoelectric point about 4.5) increased markedly after 12 to 24 h at 4 degrees C, as did mRNAs encoding four polypeptides of about 47 kilodaltons (isoelectric points ranging from 5-5.5). Incubation of Columbia callus tissue at 4 degrees C also resulted in increased levels of the mRNAs encoding the 160 kilodalton polypeptide and at least two of the 47 kilodalton polypeptides. In vivo labeling experiments using Columbia plants and callus tissue indicated that the 160 kilodalton polypeptide was synthesized in the cold and suggested that at least two of the 47 kilodalton polypeptides were produced. Other differences in polypeptide composition were also observed in the in vivo labeling experiments, some of which may be the result of posttranslational modifications of the 160 and 47 kilodalton polypeptides.