  We report an electron spin resonance (ESR) study of single crystals of the
spin-chain spin-ladder compound (Sr,La,Ca)_{14}Cu_{24}O_{41}. The data suggest
that in intrinsically hole doped Sr_{14-x}Ca_xCu_{24}O_{41} only a small amount
of holes is transferred from the chains to the ladders with increasing x,
resulting in a crossover from spin dimerized to uniform spin chains. In the
samples of La_{14-x}Ca_xCu_{24}O_{41} with reduced hole content a very broad
signal is observed in the paramagnetic state, indicative of a surprisingly
strong anisotropy of the nearest neighbor exchange in the chains.
