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Negative-resistance amplifiers that consist of a noisy negative resistance imbedded in a lossless three-terminal-pair linear network to make a two-port amplifier are analyzed. It is shown that Haus and Adler's noise measure , is not constrained to lie between two eigenvalues of a noise matrix, as is the case for bona fide two-port amplifiers. Instead, it is always equal to its optimum value, and is independent of the (lossless) imbedding network used. As a corollary of this, the noise figure of such an amplifier fails to equal its optimum value only insofar as the exchangeable gain is not high. The single value of noise measure may be computed from any simple lossless circuit at hand, or else from the exchangeable noise power of the noisy negative resistance.