The report evaluates a method to detect conjugal DNA plasmid transfer in soil slurries under various environmental conditions. Donor Pseudomonas cepacia containing pR388::Tn1721 and P. cepacia recipient cultures were co-incubated in soil slurries containing autoclaved or natural soil and treated with one or more of 14 experimental conditions. Conjugal mating frequency (transconjugants per initial donor) ranged from 4.8 X ten to the minus 1 to 1.9 X ten to the minus seventh. Highest numbers of transconjugants, 1.5 X ten to the seventh colony forming units/ml soil slurry, were observed following incubation at 35C with an enriched nutrient supplement added to the soil. Low numbers of transconjugants, ten to the third colony forming units/ml soil slurry, were observed when mating pairs were subjected to low nutrient or pH stress even though initial donor and recipient populations were maintained at high levels. The test system provides a simple way to estimate effects of changing environmental factors on plasmid transfer rates and on the survival of recombinant microorganisms. By use of soil collected from sites proposed to receive genetically engineered microorganisms, preliminary risk assessments can be obtained regarding the potential for gene transfer and microorganism survival with this soil slurry test system. (Copyright (c) Springer-Verlag New York Inc. 1989)