Experimental growth data for three human malignant tumors transplanted to nude mice of BALB/c origin are analyzed statistically in order to investigate whether they can be described according to the Gompertz function. The aim is to set up unequivocal standards for planned therapeutic experiments and to develop an essential part of the determination of proliferation parameters for the tumors. The results indicate that the course of tumor growth can be described with good approximation by the Gompertz function. A transformation of this function depicts the growth rectilinearly and appears to be suitable as a standard, e.g. in therapeutic experiments. The course of tumor growth is independent of the size of the transplant, and whether tumors are transplanted in the right or left or both flanks of the recipient mice. Furthermore, the growth does not vary in a systematic way with the number of passages in nude mice. For tumors whose growth is described according to the Gompertz function, recording of the growth of the tumor size in two dimensions is sufficient for calculating other relevant growth parameters, if the three linear tumor measurements are proportional throughout the growth period. The initial histological development of a transplanted malignant melanoma is described and used to elucidate other tumor growth measurements.