In this study we have decided to explore the diagnostic usefulness of CAST-ELISA test in patients with immediate allergic reactions caused by penicillin. Peripheral blood leukocytes were stimulated by commercially available penicillin allergens--Penicillin G, Benzyl-penicillin--Polylysine and specially performed Penicillin G conjugated to human serum albumin. Test was performed in 33 allergic persons and in 13 control healthy persons well tolerated penicillin antibiotics with negative skin tests results. CAST-ELISA test gave the positive results in 30.3% when unconjugated penicillin was used and in 18.18% with penicillin--polylysine. The best results were obtained when cells were stimulated with penicillin conjugated to human serum albumin--positive results were observed in 47.82%. Accumulate percentage of positive results was 73.91%. In our opinion CAST-ELISA test is reliable method in diagnosis of immediate allergic reactions caused by penicillin and inclusion of penicillin conjugated to human serum albumin seems to be justifiable.