Immunological methods for the study of the plasma complement system have been standardized in order to be good and reproducible indicators of some biological effects of the substances under study in in vitro experiments. The substances tested were not capable of interfering within 10 times the possible hypothetical plasma concentration reached in vivo with the function of the different reagents used in the study of complement. Five substances (Skin-ACGIH) have been studied for their effects on the complement system in vitro; four of them could be fully studied (allylic alcohol, cyclohexanone, phenol, dimethylacetamide). After this deep insight we can conclude that: 1. These substances are capable of interfering with the immune response through their complement activating capacity 2. These substances, throughout complement activation, can induce inflammation and reduction of important defensive functions that are complement mediated. 3. The results obtained encourage to study the complement system and especially CH50 in workers exposed to the selected substances in order to verify the possibility to enclose this test in the medical surveillance program.