This interlaboratory study was conducted to examine four erythrocyte protoporphyrin control materials from Aviv Biomedical, Helena Laboratories, Kaulson Laboratories, and the New York State Department of Health for use with hematofluorometers. Our principal aims were to monitor the stability of these materials at three different storage temperatures (room, refrigerator, freezer) and, where appropriate, to validate the manufacturer's target values. Measurements for the study were generated in three reference laboratories that used a total of five hematofluorometers, three from Environmental Science Associates and two from Aviv Biomedical. Each instrument was calibrated against a consensus acetic acid-ethyl acetate extraction procedure. We found the materials from Aviv to be the most stable, followed by the New York State material. However, the target values assigned by Aviv were not within the acceptable range determined by consensus. The target values assigned by Kaulson Laboratories for their materials did fall within the acceptable consensus range, but they were the least stable of the materials evaluated. The materials from Helena Laboratories were originally designed for use as calibrators with Helena's "ProtoFluor Z" hematofluorometer, which reports in different units. They were deemed unsuitable for use as control materials with the Aviv or Environmental Science Associates hematofluorometers because of the narrow range of values and the wide scatter of results.