And it came to pass at the end of forty days, that Noah opened the windows of the ark which he had made:
And he sent forth a raven, which went to and fro, until the waters were dried up from off the earth.
Also he sent forth a dove from him, to see if the waters were abated from off the face of the ground;
But the dove found no rest for the sole of her foot, and she returned unto him into the ark, for the waters were on the face of the whole earth: then he put forth his hand, and took her, and pulled her into him into the ark.
And he stayed yet another seven days; and again he sent forth the dove out of the ark;
And the dove came in to him in the evening: and lo, in her mouth was an olive leaf pluckt off; so Noah knew that the waters were abated from off the earth. (Genesis, 8, vi-xi)
The three essential components of a bioassay are present in this example: a stimulus (the depth of water); a biological test subject (the dove); and a response (the plucking of an olive leaf).
Formal bioassays were developed to study the potency of insecticides during the early twentieth century at Rothamstead Station in England. Sir R. A. Fisher and other statisticians developed experimental designs and basic statistical procedures in collaboration with toxicologists and entomologists (Bliss, 1934a,b; Finney, 1952a; Gaddum, 1933).
Since that time, the role of bioassays has been expanded considerably. Prior to the 1970s, bioassays were used only to measure the toxicity of particular chemicals in, for example, medical, pharmacological, or agricultural studies (McKee and Wolf, 1963; Sprague, 1969). During the 1970s, the EPA began to use bioassay results for particular chemicals to establish the water-quality criteria that are published in the EPA green, blue, red, and gold books (e.g., United States Environmental Protection Agency, 1973, 1986). Since then, the application of bioassays has increased to include testing the toxicity of novel chemical compounds, licensing manufactured chemicals already in use (Federal Insecticide Fungicide Rodenticide Act), measuring toxicity at superfund sites (Resource Conservation and Recovery Act), and, of most importance for this paper, testing the toxicity of effluents.
The toxicity of effluents can be assessed by exposing test organisms to a series of dilutions of an effluent and measuring the organisms' responses. There are two basic classes of effluent bioassays. Acute bioassays measure survival of the target organisms. Chronic bioassays measure their growth, reproduction, or behavior. The toxic concentration of an effluent is defined as the lowest concen-