Class 1 integrons (Int1) contribute to antibiotic multiresistance in Gram-negative bacteria. Being frequently carried by conjugative plasmids, their spread would depend to some extent on their horizontal transfer to other bacteria. This was the main issue that was addressed in this work: the analysis of Int1 lateral transfer in the presence of different antibiotic pressures. Strains from a previously obtained collection of Escherichia coli K12 carrying natural Int1+ conjugative plasmids were employed as Int1 donors in conjugation experiments. Two recipient strains were used: an E. coli K12 and an uropathogenic E. coli isolate. The four antibiotics employed to select transconjugants in LB solid medium were ampicillin, trimethoprim, sulfamethoxazole, and co-trimoxazole. For this purpose, adequate final concentrations of the three last antibiotics had to be determined. Abundant transconjugants resulted from the mating experiments and appeared in most -but not all-selective plates. In those supplemented with sulfamethoxazole or co-trimoxazole, transconjugants grew or not depending on the genetic context of the recipient strain and on the type of gene conferring sulfonamide resistance (sul1 or sul2) carried by the Int1+ plasmid. The horizontal transfer of a recombinant plasmid bearing an Int1 was also assayed by transformation and these experiments provided further information on the viability of the Int1+ clones. Overall, results point to the existence of constraints for the lateral transfer of Int1 among E. coli bacteria, which are particularly evidenced under the antibiotic pressure of sulfamethoxazole or of its combined formula co-trimoxazole. •The lateral transfer of class 1 integrons among Escherichia coli strains was assayed.•The success of Int1 transfer depends on the genetic context of the recipient strain.•The success of Int1 transfer depends on the antibiotic pressure used for selection.•The type of sul gene determines the level of sulfonamide resistance in E. coli K12.•Sulfonamide affects the viability of Int1+E. coli K12 transconjugants with only sul1.