Lactoferrin is a secondary granule protein of neutrophils. Seventy-nine systemic lupus erythematosus patients who fulfilled the ARA criteria for classification were tested for antibody against human lactoferrin (LF-ab) by ELISA. Thirty-one of these (39.2%) demonstrated elevated levels. There was significant correlation between LF-ab positivity and disease duration. Clinical flare was common with positive LF-ab (P less than 0.05). Disease manifestations were independent of antibody status except for an increased incidence of lymphadenopathy and crescentic gomerulonephritis among those who had LF-ab. No consistent immunofluorescence pattern could be demonstrated on alcohol-fixed neutrophils for the LF-ab positive sera. It is suggested that LF-ab is related to lupus activity, and can be useful as a marker for disease monitoring.