The effectiveness of currently used pesticide mixing-loading and application equipment in preventing workers from being exposed to cholinesterase (ChE) inhibiting pesticides was measured. Blood samples from workers involved in the application of pesticides in Monterey and Imperial counties of California were analyzed for ChE activity. The analyses indicated that only the mean red cell activity of mixer-loaders was significantly less than controls. Seasonal variations in ChE activity were noted for workers in Imperial County between January and August. These variations were apparently related to the toxicity of the pesticides used. A prototype, closed-transfer system used in Imperial County did not substantially reduce pesticide residues in the air around mixing-loading sites or prevent a reduction in ChE activity over that of open-transfer systems. This was attributed to inadequate training in the use of the new equipment on the part of the mixer-loaders and improperly maintained equipment. A protytype closed-transfer system used in Monterey County appeared to give some protection to mixer-loader applicators during the application season; however, the blood ChE activities of two mixer-loaders using another prototype closed system in Monterey County were severely depressed.