Small-cell lung cancer (SCLC) lines are distinguished from non-small-cell lung cancer (NSCLC) lines by their growth in floating aggregates, in contrast to the adherent monolayers formed by NSCLC cells in culture. Of 50 well-characterized SCLC lines recently described by the National Cancer Institute (NCI)-Navy Medical Oncology Branch, only four variant cell lines (SCLC-v) grew as adherent monolayers. One line, NCI-H446, was unique in growing long-term with coexisting floating and surface adherent subpopulations. We have physically segregated these two populations over many passages in vitro to enrich for relatively pure cultures of floating and adherent cells. No differences in c-myc expression, keratin pattern, or cytogenetic appearance were found between the adherent and floating sublines. However, expression of the neuroendocrine marker neuron-specific enolase in the floating cells was three times that found in the adherent cells. The floating subline also had much greater surface expression of neuroendocrine tumor antigens detected by monoclonal antibodies UJ13A and HNK-1, which have been recently shown to detect the neural cell adhesion molecule (NCAM) on SCLC cells. Two other adherent SCLC-v lines were also found to be unreactive with UJ13A and HNK-1, generalizing the association between NCAM expression and the growth of most SCLC cultures as floating aggregates. In conclusion, we have an interesting model to study expression of NCAM as related to the adhesive properties of SCLC cells. (J. Clin. Invest. 1990Invest. . 86:1848Invest. -1854 Key words: lung cancer * neural cell adhesion molecule * neuron-specific enolase Introduction A striking difference between small-cell lung cancer (SCLC)'