Twelve papers in this series were derived from two conference sessions focusing on causality in field studies. Eight of these papers involve case studies examining biological effects of chemical contaminants in field situations. Using a weight-of-evidence approach, these case studies were evaluated against seven proposed criteria for establishing causality. The seven criteria were: strength of association; consistency of association; specificity of association; time order; biological gradient; experimental evidence; and biological plausibility. One of these seven criteria, 'specificity of association' was found to be of little utility for establishing causality in these field studies. The case studies are presented in approximate order of increasing levels of biological organization (i.e., going from endpoints at the suborganismal level to endpoints at the population or community level). In case studies examining higher levels of biological organization, it appears that the 'biological gradient' criterion was also not useful in establishing causality. These results, together with suggestions from other papers in the series, are used to recommend a set of modified criteria for establishing causality in field studies of the biological effects of chemical contaminants.