The subcutaneous administration of epoetin alfa preparations may cause pain at the injection site. To identify the pain-causing substance in these formulations we performed two double-blind, placebo-controlled, randomized order, cross-over studies. Differences in pain experienced after subcutaneous injection of an epoetin alfa solution and its vehicle were assessed in 36 patients. The vehicle and its component parts, albumin and citrate, were compared in 36 volunteers. Normal saline served as a placebo control in both studies. Pain scores were obtained from visual analogue pain scales with no divisions and from five point verbal descriptive pain scales. Both the epoetin alfa solution and its vehicle caused significantly more pain than normal saline (P < 0.0001) in the patients studied. In volunteers the pain scores with the vehicle or its citrate component were significantly higher (P < 0.0001) when compared with normal saline or with the albumin component of the vehicle. In conclusion, the local pain experienced after subcutaneous administration of epoetin alfa preparations is mainly caused by the citrate component of the buffered solution. Epoetin alfa and the albumin component of the preparation do not play a role in this phenomenon.