Samples (801) of fluid market milk (about 50 from each of the 16 Food and Drug Inspection districts) were collected in the fall of 1955 and tested for residues of chlorinated organic pesticides by bioassay with houseflies. Of the samples 62% reacted positive (mortality; characteristic toxic symptoms). Toxic residues occurred mostly in trace amounts that were not identified specifically. Samples (106) showing the highest fly kill, selected on the basis of about 10 from each district, were further examined by paper chromatography. BHC, DDT, lindane, DDD, DDE, and methoxychlor were identified in the residues in this order of occurrence. Of these high mortality samples 53% gave a positive Schecter-Haller test for DDT and related compounds. The highest result by this test was 1.46 ppm as DDT. Organic phosphate pesticides were not detected (all 801 samples) by an in vitro cholinesterase inhibition test.