This thesis concerns a melanoma-derived growth regulatory factor that inhibited proliferation of several malignant human cell lines, and, in particular, a line designated UCT-BR-1, which was derived from a human breast cancer metastasis. The work is presented in four chapters. Chapter 1 provides a review of the relevant literature at the time of writing; Chapters 2 and 3 describe the experimental work that was done; and in Chapter 4 I discuss the implications of my results for current and future work in growth factors. Experimental results are presented as Charts (which may be Figures or Tables) and the methods and experimental protocols that I used are described in the Chart legends and not in the main text of the thesis. The Appendix contains details of the tissue culture techniques and descriptions of the cell lines that were used. Sources of the various laboratory materials as well as the methods that were employed for the more routine procedures are also described in the appendix.