Lithologic, hydraulic, geophysical, and water-quality data collected at the McKay Creek subsurface waste-injection test site in Pinellas County, Florida, are reported. Data were collected to determine the possibility of subsurface injection of waste-treatment plant effluent. One exploratory hole, one test injection well, and eight observation wells were constructed between May 1973 and February 1976. The exploratory hole was drilled to a depth of 1,750 feet below land surface; the test injection well is open in dolomite between 952 and 1 ,040 feet; and the observation wells are open to intervals above , in, and below the test injection zone. The lithology of the upper 100 feet is predominantly clay. From 100 to 1,750 feet below land surface, limestone and dolomite predominate. Gypsum is present 1,210 feet below land surface. Laboratory analyses of cores taken during drilling are given for vertical intrinsic permeability, porosity, interval transit time, and compressibility. Specific capacities tested during drilling range from 4 to 2,500 gallons per minute per foot of drawdown. An 83-hour withdrawal test at 4,180 gallons per minute and a 2-month injection test at 650 gallons per minute were run. Small water-quality changes were observed in one observation well immediately above the test injection zone during and after the injection test. Formation water in all of the wells with the exception of the shallowest observation wells is saline. The vertical position of saltwater is estimated to be at about 280 feet below land surface. Thirteen wells within a 1-mile radius of the test site were located and sampled for water quality. (USGS)