The solid solution alloys formed by the incomplete d-shell metals in groups 4, 5, 6, and 7 have been tested for superconductivity down to 1°K. For alloys formed between neighboring elements in a given row of the periodic table, two transition temperature maxima are observed with valence numbers approximately equal to 4.7 and 6.4, respectively, the only exception being the first long period, in which the upper maximum is absent. Similar maxima occur when the constituent elements are selected from different rows of the periodic table, thus confirming the dominant role of the d-shell electrons. It is known that the normal density-of-states function, N(0), passes through a series of maxima as the d-shell is filled up, two of these peaks lying at about the same composition as the two transition temperature peaks observed in the present work. The relationship of Tc to N(0) for the transition metal alloys is discussed. Transition temperature data are also presented for alloys composed of neighboring elements in a given column of the periodic table. In this case, the form of the relationship between Tc and electronic or lattice properties is still obscure.