Conjugates of horseradish peroxidase with antibodies (anti-human IgG (H + L)) or their Fab' fragments were prepared according to the newest modification of the periodate (P-) method or the two-step glutaraldehyde (G-) method. The conjugates were analysed by gel chromatography and subsequently tested in three different applications. For tissue immunohistochemistry on sections of lupus erythematosus skin, G-conjugates were preferred to polymeric P-conjugates. In ELISA for detection of human antibodies against penicillin P-conjugates were superior to G-conjugates. For the detection of surface Ig on lymphoid cells both types of conjugate were more or less equally suitable. A scheme for the most suitable combinations of method of preparation and field of application is given.