AIMS--To assess the effect of microwave heating on immunohistochemical staining of CD15 and CD30 antigens in Hodgkin's disease tissue samples. METHODS--Formalin fixed, paraffin wax embedded sections from 20 cases of Hodgkin's disease (six mixed cellularity, 14 nodular sclerosis) were immunostained for CD15, using two antibodies (DAKO-M1 and Leu-M1) and for CD30 using the antibody Ber-H2. The staining was carried out by conventional techniques which included pretreatment of sections with trypsin and on untreated sections following heating with microwaves. With antibody Leu-M1 an additional method, using a specific antimouse IgM bridge both with and without microwave heating, was also included. The results for each method were compared by counting positively stained Reed-Sternberg cells and estimating the staining intensity. RESULTS--Microwave heating resulted in a substantial increase in the number of cells stained with antibodies to CD15 and also in the staining intensity. The best results were obtained using Leu-M1 with specific rabbit anti-mouse IgM bridge and microwave heating. Dramatic enhancement of the staining of Reed-Sternberg cells for CD30 was achieved following microwave heating, together with disappearance of the non-specific staining of plasma cells. CONCLUSION--Microwave heating is strongly recommended for the immunohistochemical staining of CD15 and CD30 expressed by Reed-Sternberg cells in Hodgkin's disease.