Flower showing that therapeutic doses of aspirin reduced prostaglandin E and F in human semen. His early research included looking at the veins of the back of the hand, and the behaviour of human peripheral blood vessels, being first to develop methods for studying how human veins respond to drugs and natural mediators in vivo and developed the idea that human veins and arterioles have very different pharmacologies. Through this work he classified vasoactive drugs as veno-selective, arteriolar-selective or non-selective, and that depending on their selectivity they would have different effects in disease. He later looked at the role of nitric oxide and angiotensin converting enzyme in controlling blood vessel tone. In 1987, he set out with Patrick Vallance to investigate whether human blood vessels demonstrated endothelium-dependent relaxation, a term coined in 1980 by Robert F. Furchgott and John V. Zawadzki after discovering that a large blood vessel would not relax when its single-layered inner most lining was removed. They subsequently showed that the occurrence was mediated by what they called endothelium-derived relaxing factor, later found to be nitric oxide, and it was subsequently shown to occur in a variety of animals. Using veins from the back of a human hand, Collier and his team reproduced Furchgott and Zawadzki's findings. They then demonstrated that the human arterial vasculature is actively dilated by a continuous release of nitric oxide. In 1996, Vallance presented his and Collier's findings at the Goulstonian Lecture of the Royal College of Physicians, where he gave details of the connection between nitric oxide and blood pressure. Commission for Racial Equality Collier became a whistleblower in 1986 when, following his report published jointly with psychiatrist A. W. Burke, he informed the Commission for Racial Equality (CRE) that software used for medical-school admissions selection at St George's had been developed to intentionally discriminate against women and ethnic minorities, by mimicking methods of previous admissions officers. It was creating a lower score for women and those with non-European (Asian, African and Arabic) names so reducing their chance of being called for interview. Collier's findings led to an enquiry, both internal by St. George's and external by the CRE, and a subsequent report was published entitled Medical School Admissions: Report of a formal investigation into St. George's Hospital Medical School (1988). Like Collier and Burke, the report also raised the question of what might be happening in other London medical schools;