Thirty-five pulmonary carcinomas were studied retrospectively with monoclonal antibody (MCA) 44-3A6 raised against a human adenocarcinoma cell line. The antibody was applied to cytologic smears of bronchial brushings originally stained with the Papanicolau method, and to conventional tissue sections. Ten of 12 adenocarcinomas (ADC) immunostained strongly in sections and smears, as did five of seven large-cell "undifferentiated" carcinomas (LCUC). Eight neuroendocrine carcinomas (NEC) and eight squamous-cell carcinomas (SCC) were negative, except for rare weakly positive foci. We conclude that MCA 44-3A6 can be effectively applied on cytologic smears, and that it could be valuable in the precise classification of pulmonary carcinomas. The immunoreactivity of the ADC and SCC was predictable. Positive immunostaining in some LCUC confirms that these constitute a heterogeneus tumor class that includes cases that are phenotypically ADC despite the absence of obvious glands. Occasional immunostaining in NEC suggests focal exocrine differentiation as previously noted by electron microscopy.