The wide-spread use of biologically active chemicals, including pesticides and antibiotics, involves a potential health hazard. Measurement of residues of these materials in foods is a necessary part of the evaluation of this hazard. Although the organic phosphorus insecticides are, in general, more toxic, the chlorinated hydrocarbon compounds present more of a residue hazard due to their greater stability. The major problem associated with antibiotics in food materials involves residues in milk. Analytical methods applicable to some of the common pesticides and antibiotics are reviewed.