Secondary metabolites are the microbial products which are produced during the idiophase of microbial growth. These are secreted when depletion of one or more nutrients is caused in the culture medium. Secondary metabolites are not further required by the micro-organisms for their growth. Secondary metabolites e.g. alkaloids, essential oils, pigments, etc. from cultured plant cells are described under Immobilized cell culture. But toxins and antibiotics of microbial origin are described in detail in Secondary Metabolites.