Results from 413 photographic observations of the minor planet (1566) Icarus, including 342 from the 1968 close approach, were analyzed to improve the determination of Icarus' orbit and to test its consistency with the theory of general relativity. Introducing a parameter X which would assume the value unity were general relativity correct and zero were Newtonian theory valid, we find X = 0.95+0.08. (The uncertainty given is the formal standard error when that for each observation is assumed to be one arc second.) The value obtained for X is, however, influenced significantly by the model employed to counteract inadequacies in the astrographic catalogues used in the reductions of the photographic plates. Corresponding sensitivity studies indicate that, on the basis of Icarus data alone, the real uncertainty in X is about 0.2. If the relevant stellar positions are ever improved commensurate with the intrinsic accuracy of the location of Icarus with respect to them, the value of X determined from the Icarus observations through 1968 will have an un- certainty of only about 0.05 provided also that the contributions of the solar quadrupole moment are either known or negligible. The Icarus data, as we had predicted previously, are of marginal use for the determina- tion of the solar quadrupole moment and cannot be used to improve estimates of other astronomical con- stants such as the mass of Mercury whose uncertainty on the basis of the Icarus data alone is about 500000 (in terms of inverse solar masses), i.e., more than 20 times greater than the uncertainty deduced from prior analyses of radar data.