The effect of suramin on the in vivo growth of 4 human malignant melanoma xenotransplantation lines in nude mice was studies. The drug (multiple i.p. injections of 80 mg/kg) was well tolerated by the animals, however, no tumor regressions were seen. All 8 different sets of tumor growth data (4 treated and 4 control groups of 8 tumor-bearing animals each) fitted well to the Gompertz difference equation, which enables to precisely analyse the pattern of tumor growth. The results show that suramin treatment leads to a statistically significant inhibition of the intrinsic growth rate of all tumors and also a simultaneous partial inhibition of the growth deceleration rate in three of the 4 tumor lines. As result of this complex alteration the growth of most melanoma lines presently studied is ultimately stimulated by suramin treatment in vivo. However, one recently established line is an exception: in this tumor suramin did not significantly affect growth retardation and thus, led to an overall growth inhibition. One possible interpretation might be, that suramin simultaneously inhibits both the positive and the negative growth regulatory mechanisms of the tumors to different degrees leading to a new balance between them, which is expressed in a different growth pattern.