We studied the expression of peptidylarginine deiminases (EC 126.96.36.199) in an immortalized newborn rat keratinocyte cell line. No measurable enzyme activities were noted in either growing or confluent cultures. The enzyme activity was increased by all-trans retinoic acid in dose- and time-dependent manners. The enzyme activity was resolved into two peaks by anion exchange chromatography. The minor peak resembled enzyme preparations obtained from the epidermis in earlier studies. The major peak was indistinguishable from rat muscle peptidylarginine deiminase in the chromatographic and Western blotting profiles. Northern blot hybridization showed a major band in retinoic acid-treated cells migrating slightly behind muscle peptidylarginine deiminase mRNA.