A "well behaving" property is defined for each line (in general a bundle of wires) connecting two combinational logic cells. This property can be easily taken into account in the design process of the circuit. It is shown that all tree-structured circuits of combinational logic cells are easily testable for multiple faults if each line is "well behaving". The size of the test set increases only linearly as the number of cells increases. It is also shown that if no line is well-behaving then only the trivial (exhaustive) test set exists for the circuit, which increases exponentially as the number of cells increases. If there are well-behaving as well as non-well-behaving lines in the circuit, then the size of the test set increases exponentially with the sizes of the non-well-behaving subtrees and linearly with the number of such subtrees.