Antibodies against interleukin-1 alpha (aAb-IL-1 alpha) have previously been demonstrated in sera of healthy adults and in patients with inflammatory diseases. In the present investigation the occurrence of aAb-IL-1 alpha was examined by second antibody precipitation technique in sera from umbilical cords (n = 11), children (n = 45), and adults (n = 20), as well as in 51 patients with juvenile chronic arthritis (JCA) of pauciarticular (n = 34), polyarticular (n = 8), or systemic onset type (n = 9). RESULTS. The frequency of positive sera was significantly lower in children than in cord blood and adults as were the levels of aAb-IL-1 alpha (p <0.0001). In the non-neonatal group of individuals the levels of aAb-IL-1 alpha correlated positively with age (r = 0.394, p = 0.0015). The frequency of sera positive for aAb-IL-1 alpha was higher in the JCA patients, as were the levels (p <0.005), and correlated positively with disease activity as evaluated by joint score and visual analogue score.