By this time, his research interests had turned to human tumor cultures, principally culturing early passage cell lines from brain tumors to develop a predictive test for chemosensitivity. He spent a few months in Dr. Richard Ham's laboratory, testing the growth of glioma cell lines in serum-free medium, finding considerable variation among glioma lines tested, but gaining a greater understanding of the underlying principles.

His interests then turned to examining the effects of glucocorticoids on glioma and lung carcinoma in vitro. These studies implicated cell surface modification in the response to glucocorticoids and initiated a return of interest to cell-cell interaction in an attempt to regulate malignancy by inducing expression of differentiation. Studies with lung carcinoma confirmed that lung fibroblasts produced a factor which induced differentiation in the lung carcinoma cell line A549 and that this effect could be reproduced with cytokines such as oncostatin M, interleukin-6 and interferon b. The action of conditioned medium, and of the cytokines, required dexamethasone, a synthetic glucocorticoid analogue, and it was shown that this was due to induction of synthesis by the A549 of a specific heparan sulphate which activated the fibroblast conditioned medium and the cytokines, possibly by stabilization or translocation to the high affinity receptor.

His interests are now focused on cell culture in general, and on cell-cell interaction and paracrine control of cell differentiation, in particular. He teaches a number of international basic and specialized cell culture courses. He is the author of Culture of Animal Cells, a Manual of Basic Technique, and has edited, or co-edited six other books on specialized cell culture. He has also written numerous reviews and original articles in the areas of cell culture, cytotoxicity assay, and induction of differentiation in vitro.