The relationship between sewage effluent particulates which reach bathing beaches and swimming-associated health effects was examined. A differential filtration procedure was used to quantify the density of particles greater than 3 microns in size carrying viable Escherichia coli cells which served as a surrogate pathogen and the average number of Escherichia coli per particle. Total particle density also was measured. Health effects were determined using a prospective epidemiological approach. The results of this two-year study show that there is an apparent relationship between swimming-associated illness rates and the density of microbial-bearing particulates at a freshwater beach. No relationship was observed between swimming-associated health effects and the total density of particles or the average number of viable bacterial units per particle. These results suggest that pathogens associated with illness in swimmers may circumvent disinfection and natural purification processes and reach bathing beaches by virtue of their being encapsulated in fecal particulates.