The influence of total organic carbon (TOC), pH, and mating temperature on transfer of chromium-resistant plasmid between Escherichia coli strains in terms of variation in the number of transconjugants formed and variation in transfer frequency was investigated. In vitro transfer was studied in five chromate-tolerant E. coli strains isolated from tannery effluent using E. coli K12 J62 (Nal(r) Lac(-)) as a recipient. Conjugal transfer of different selection markers was observed in three strains. The study was carried out in sterile wastewater. A gradual decrease was observed both in the number of transconjugants and in transfer frequencies as the concentration of TOC in the mating medium descended from 10,095 to 1.2 mg of C/L, obtaining the maximum values with a TOC concentration of 10,095 mg of C/L. The number of transconjugants and the transfer frequency were maximum at 30degreesC. However, neither the transfer frequency nor the transconjugant number varied significantly in the range of pHs assayed. The strains were also found resistant to different heavy metals and antibiotics. Curing of these strains resulted in loss of one or more resistance markers indicating the plasmid-borne resistance. It is inferred that plasmid transfer by conjugation occurs in wastewater bodies within a wide range of conditions.