Abstract Acholeplasma laidlawii wild type and a mutant strain, comr-328, characterized by its resistance to complement-mediated killing, were examined for the effects of variations in their lipid fatty acyl moieties on membrane fluidity and the susceptibility of the cells to lysis by complement. Both cell types were cultured in media containing stearate, palmitate, or an equimolar mixture of palmitate and oleate. When stearate or palmitate served as the major membrane fatty acyl constituent, the sensitivity of the cells to complement-mediated killing via both the classical complement pathway and reactive lysis was increased, whereas the fluidity of the respective cell membranes was decreased. By comparison, when oleate was a major membrane fatty acyl moiety along with palmitate, the cell's sensitivity to complement-mediated killing decreased, whereas membrane fluidity increased. This pattern was observed for both strains although it was more pronounced in the wild-type strain than in comr-328. Although there appears to be a correlation between membrane fatty acyl composition, membrane fluidity, and complement-mediated killing in A. laidlawii, this correlation is obscure since the presence of cholesterol in the membrane, although affecting membrane fluidity, apparently does not alter the response of wild-type cells to lysis via the classical pathway.