This study has addressed the question whether there is a difference in resistance to Triton X-100 penetration of sphingomyelin and phosphatidylcholine membranes. We have studied this by measuring the penetration of Triton X-100 into monolayers of either D-erythro-N-palmitoylsphingomyelin or dipalmitoylphosphatidylcholine (DPPC) at constant lateral surface pressure and 22 degreesC. In addition, we also compared how Triton X-100 penetrated into egg phosphatidylcholine and D-erythro-24:1(Delta 15c) sphingomyelin monolayers and how the presence of cholesterol affected the penetration process. In bilayer membranes we assessed Triton X-100 insertion and solubilization by studying the thermodynamics of the partition process and determining the detergent:ratio at which the solubilization is initialized. The penetration of Triton X-100 from the subphase into the monolayers was concentration and surface pressure dependent. The penetration increased linearly with time and concentration for DPPC monolayers, but with sphingomyelin monolayers, the response was clearly biphasic and faster. If the penetration was allowed to proceed, the monolayers eventually became unstable and collapsed (at collapse the detergent-to-lipid ratio was about 1:4 in the monolayer). Inclusion of 30 mol % cholesterol in the monolayers made them more resistant to Triton X-100 penetration, irrespective of the host phospholipid type. In bilayers the partition coefficient (K) becomes lower and the enthalpy change (DeltaH) higher for partitioning of Triton X-100 into D-erythro-N-palmitoylsphingomyelin as compared to into DPPC. Onset of solubilization was reached at lower detergent:lipid ratios in D-erythro-N-palmitoylsphingomyelin than in DPPC vesicles. In conclusion, we have observed that D-erythro-N-palmitoylsphingomyelin monolayers; do not show increased resistance toward Triton X-100 penetration as compared with membranes prepared from an acyl-matched glycerophospholipid, at least not at room temperature and not even when cholesterol is present. Sphingomyelin bilayers are more susceptible to Triton X-100 solubilization than phosphatidylcholine bilayers, although K was lower for Triton X-100 into sphingomyelin bilayers.