Twelve workers with hard metal asthma diagnosed on the basis of peak flow diaries and positive bronchial reactions to cobalt chloride (CoCl2) were studied for sensitization by detection of specific antibodies to radioactive cobalt (57Co), cobalt-conjugated human serum albumin (Co-HSA) and cobalt-conjugated exchange resin (Co-resin). Their IgE titres ranged from 73 to 1500 IU/ml and eight were atopic individuals. Sixty serum samples from asthmatic patients with IgE titres of 14–4300 IU/ml were studied as controls in all tests. Eleven of twelve subject sera that selectively bound to 57Co after incubation with saturated ammonium sulphate (>232 c.p.m., P<0.01) were divided into three groups: (1) six sera showing evidence of specific IgE antibodies to Co-HSA (>673 c.p.m., P<0.01) without those to Co-resin; (2) one serum giving a positive radio-allergosorbent test (RAST) only to Co-resin (>417 c.p.m., P<0.01), and (3) four sera that were negative for two antigenic agents (Co-HSA, Co-resin). These results suggest that the subjects had occupational asthma due to hard metal exposure from cobalt sensitivity. An immuno-allergic mechanism mediated by specific IgE antibodies to cobalt was confirmed to be responsible for the development of hard metal asthma, with the possibility of some role of the reaction without reagins.