A summary of preliminary studies designed to investigate the possibility of utilizing vertical starch gel electrophoresis as a test for the presence of some of the organophosphorus pesticides in plant extracts, without prior cleanup of the extracts, is presented. There is a considerable variation in the quantity and number of some esterases from different animals. This difference has been noted in liver extracts of rats, rabbits, and fish and in the serum esterases of the rats and rabbits. These observations indicate the necessity of a control extract for each pattern. The esterase bands in the control liver were numbered from 1 to 14, where 14 is the slowest moving band near the slot or origin. The results to date indicate that the organophosphates discussed in this report may be placed into four groups as follows: 1) Malathion, Di-Syston, Ethion, Rogor, Guthion, Co-Ral, ronnel and schradan. These show no apparent inhibition of rabbit liver esterases under the conditions employed and at the three levels used (0.001, 0.0001, and 0.00001M). 2) Dipterex and Phosdrin. These inhibit only the three leading bands, 1, 2, and 3 at all three levels. 3) TEPP, Dibrom, Systox, Ruelene, parathion, Sevin, and methyl parathion. These inhibit the leading bands as well as some of the other bands at the 0.001M level. 4) Nemacide, Trithion, EPN, Thimet, and Diazinon. These inhibit more than one esterase, but none of the five inhibits the three leading bands. The degree of inhibition and the number of esterases inhibited were dependent on the concentration of the pesticide with the exception of TEPP and Phosdrin. Results indicate that some of the above pesticides are more potent inhibitors of some esterases after bromine treatment, while others are not. This suggests that liver extract may contain enzyme systems capable of converting some of the pesticides to active inhibitors before inhibition of the esterase actually takes place. Liver homogenates may also contain enzymes which degrade the pesticide before the inhibitory reaction takes place.