Abstract
Two two-dimensional Ising models with two- and four-spin interactions are studied. Evidence from high temperature series expansions suggests that the susceptibility exponent gamma remains constant at 1.75, independent of J4, the strength of the four- spin interaction. This is in agreement with the general 'universality' hypothesis. Mean field theory predicts that for J4 large enough the transition becomes first-order, but we find no evidence for this from the series expansions.