In screening a rat mucosa cDNA subtraction library, a clone that exhibited a remarkable degree of homology with a previously described cDNA from the green garden pea, designated the 26g pea turgor protein, was found. A partial cDNA sequence from rat and a complete cDNA sequence from human were obtained. The deduced human protein had a molecular weight of 55,285 and was designated antiquitin because of its remarkable level of conservation through evolution. Human antiquitin was 60% homologous to the green pea 26g with only a single amino acid gap in each sequence. The 66 amino acids at the carboxyl ends of the human antiquitin and pea 26g proteins were 86% identical, and one segment of 52 amino acids was 92% identical. A similar partial sequence encoding 164 amino acids has been detected in Caenorhabditis elegans. Yeast DNA was found to have sequences that hybridize with a human antiquitin probe on Southern blotting. Analysis of the amount of mRNA in various rat and human tissues indicated that the largest amounts were found in rat kidney and liver and in cultured human hepatoma cells. Only minimal amounts were detected in human peripheral blood leukocytes, rat lung, or cultured human fibroblasts. Attempts to induce the mRNA by heat-shock, dehydration, ionizing irradiation, or treatment with iron, t-butylhydroperoxide, or glucocorticoids were unsuccessful. The function of the protein remains unknown.