Twenty-one workers with hard metal asthma, including eight atopics, diagnosed on the basis of peak flow diaries and positive reaction to cobalt chloride (CoCl2) and/or nickel sulphate (NiSO4) in skin and provocation tests were studied for sensitization by detection of specific antibodies to nickel-conjugated human serum albumin (Ni-HSA), and nickel-conjugated exchange resin (Ni-resin). Their results were compared with those of sera obtained from control sera from 60 asthmatic patients and pair-matched asymptomatic control workers in the hard metal plant. In the RASTs (radioallergosorbent tests), sera from the same six subjects developed positive reactions both to Ni-HSA (RAST index > 2.0, P < 0.01) and Ni-resin (RAST index > 2.0, P < 0.01), while the counts measured for the others of the 15 subjects (RAST index < 1.52) were about the same as those for control groups (RAST index < 1.58). Subject HSA RAST and resin RAST results (378.52 c.p.m. in HSA RAST and 324.56 in the resin RAST) were about the same as those of the control sera (388.65 c.p.m. and 398.59 c.p.m., respectively). There was no difference in the prevalence of smoking habit and high IgE between Ni-RAST positive and negative subjects. However, subjects with simultaneous sensitivity to nickel and/or cobalt still developed asthmatic attacks following medications, while those without sensitivity to these metals were almost symptom free. The positive sera had simultaneous sensitivity to both cobalt and nickel, suggesting the presence in them of specific IgE antibodies to nickel playing some role in the aetiology of hard metal asthma.