The demonstrated relationship between carcinogenicity of a chemical compound in mammals and its tendency to cause prophage induction in bacteria provides a method for biologically based carcinogen screening. Because of the need for this type of screening and the abundance of lysogens in the marine environment, 14 isolates were evaluated for the degree of prophage induction in exponentially growing cultures in the presence of a known mutagen (0.5 microg/ml mitomycin C). Assays were performed both in liquid culture and in microtiter plates. Virus-like particles were enumerated by epifluorescence microscopy after staining with SYBR-Gold. Two isolates designated P94-4B3 (identified as Halomonas aquamarina) and P94-4S3 (identified as Pseudomonas aeruginosa) were further evaluated for potential use. Because of the rapid growth, larger size of its virus-like particles, and linear response to increasing dose of mitomycin C, the P. aeruginosa st. P94-4S3 was determined a better candidate for the marine prophage induction assay (MPIA). The Pseudomonas isolate was then used in several experiments for the development and optimization of the MPIA procedure. Initial screenings of the MPIA were also performed with selected environmental pollutants.