Escherichia coli clones containing hybrid phasmids with the inserts of B. pertussis DNA were obtained with the use of a phasmid vector. The total amount of the clones thus obtained was 97,000, which considerably exceeded the volume of the clone library necessary for the detection of individual genes with probability approximating 1. The hybrid plasmids were shown to contain 6-19 kilobases. The screening of the clone library was carried out by means of the enzyme immunoassay (EIA). The assay was aimed at detecting clones containing the genes of the subunits of B. pertussis lymphocytosis-stimulating factor (LSF). The EIA techniques used in this investigation were based on the capacity of LSF for binding with fetuin. Six clones giving positive response were detected. These data suggest the presence and expression of the genes controlling the synthesis of the antigenic determinants of LSF in E. coli cells.