Serological detection of  IgG antibodies to antigens one soil isolate 0f thermophilic actinomycetes belonged to the species Thermoactinomyces vulgaris in  (70) sera of patients with farmer's lung disease and hay fever patients compared with control normal peoples in the same environmental condition  were carried out using  ELISA method. The results indicated that (19) sera from symptomatic patients having  IgG level between (1.007-1.626 ) I.U/ml, four of them having  farmer's lung disease and the remaining have hay fever as diagnosed by clinical . The IgG level  between (0.293-0.944) I.U/ml indicated negative and which was found  in(16) patients  two of them having farmer's lung disease and the remaning have hay fever. The IgG level of  normal people were devided in two groups:  first account positive control  was between (1.089-2.147) I.U/ml and the second account negative was between (0.819-0.959) I.U/ml. The present study has demonstrated the usefulness of estimation  of specific IgG antibodies activity  to thermophilic  actinomyces by ELISA for the diagnosis of farmer's lung disease as  a screening test of large number of samples with sufficient high sensitivity.