The addition of exogenous cyclic nucleotides to cultured neoplastic cells has been reported by others to cause changes in growth properties and cellular morphology. We have studied the in vitro morphologic response to exogeneous dibutyryl adenosine 3':5'-cyclic monophosphate (DBcAMP) of twelve permanent cell lines derived from human gliomas, thirteen sublines and clonal lines derived from human gliomas, and nine cell lines derived from neoplastic and non-neoplastic human tissue from sources other than gliomas. Replicate samples from each cell line were evaluated during log phase growth by counting cells with cytoplasmic extensions greater than two times the diameter of the cell body and at saturation density for the percentage of cells morphologically responding to DBcAMP. The mean percentage of glioma-derived cells responding ranged from 5.0 to 97.3 percent during log phase growth and from 0 to 100 percent at saturation density; from 0 to 95.0 percent of non-glioma derived cells responded. The percentages of responding cells from gliomas and controls for any given line was reproducible within and between triplicate observations. Further evaluation of the twelve permanent human glioma-derived cell lines during log phase growth and at confluence delineated two clearly separate groups of cell lines. During log phase growth, eight of twelve lines had between 19.7 and 88.0 percent responding cells. At confluence, these same cell lines had between 25 and 100 percent responding cells. The other four cell lines had, respectively, 5.7 to 7.7 percent and 0 to 10 percent responding cells. The spectrum of percentage of responding cells and the variable nature of the response elicited provide evidence for the heterogeneity of the cell populations present within human glioma-derived cultured cell lines. The in vitro morphologic response to DBcAMP was reproducible and could be quantitated, but the nature and mechanism of the response to DBcAMP, whether "toxic," "differentiating," or otherwise, could not be determined from these studies.