We have performed magnetization measurements as a function of magnetic fields up to 4 kOe on Bi1.7Pb0.3Sr2Ca2-xRExCu3O10+y (RE: Gd-0.01, SM0.03), at different fixed temperatures T < T-c in the range 15 less than or equal to T less than or equal to 75 K. We have observed three important features. Firstly, the M-H curves of both the Sm- and Gd-doped samples are similar and each shows a minimum of M at a certain critical field H-c, above which there appears to be a monotonic decrease in magnitude. Secondly, the magnitude of the initial susceptibility dM / dH, related to the superconducting volume, decreases with increasing temperature. In other words, the decrease of \dM / dH\ with T indicates that the superconducting regions reduce in size and the paramagnetic domains probably grow at the expense of superconductivity. The third feature is the fast decrease of hysteresis loops in both the Gd and Sm doped samples with increasing temperature, which implies existence of flux pinning centres. Thus the results show that the Ca2+ --> Sm3+ and Gd3+ substitution decreases the hole carrier concentration and hence the volume fraction of the superconducting phase.