High frequency conjugal transfer systems were analysed in lactococci using by 5 donor and two recipient strains with different phenotypic characters. High frequency transfer of plasmids was found to be associated with the formation of aggregation structures in donor-recipient strain mixtures in the conjugation experiments. Aggregation abilities of Lactococcus lactis strain mixtures were determined to be controlled with a two element-system one of which was encoded by lactose plasmids (Clu) and the other by chromosomal DNA (Agg). Conjugal transfer frequencies of plasmids were obtained at high levels such as 4.2 x 10(-2)-1.5 x 10(-4) per donor cell when aggregate forming donor-recipient mixtures were used. Plasmid stabilities were determined between 82-98 % in conjugants obtained from conjugation experiments in which L. lactis subsp. lactis MLL40-29 was used as recipient. A reduction in the production of a 48.9 kDa protein was determined with the disappearance of the clumping phenotype.