An assay has been developed for Bordetella pertussis heat-labile toxin (HLT) based on morphological alterations in certain human embryonic lung (HEL) cell lines. Eighteen cell lines from human and other sources were tested but only two, MRC-5 and HELu2, were responsive to HLT. Confluent monolayers of the cells contracted within 24 h of exposure to the toxin, but without loss of viability during incubation for a further 3 days. The effect of HLT was quantitated by scoring the extent of morphological change, and by the decrease in Giemsa staining of the cell monolayers, as measured on an ELISA plate reader. This cell culture assay for HLT was more sensitive than lethality titration in mice but the dose-response curve had a lower slope. The specificity of the response was established by comparing unheated HLT with HLT heated at 56 degrees C, and with extracts from transposon-insertion mutants of B. pertussis which were deficient in HLT. Purified preparations of pertussis toxin and B. pertussis lipopolysaccharide gave no morphological response even at high doses.