The properties of the cation-permeable excitability-inducing material (EIM) channels in lipid bilayer membranes have been investigated using membranes penetrated by only one to four active channel units. Under these conditions, individual channels may be observed to undergo on-off conductance transitions that may be correlated with the negative resistance characteristics of membranes having a large number of channels.In many lipid membrane systems, EIM channels have several conductance levels for each channel. Such channels may open or close with either a single, large conductance step or in several smaller jumps having a sum equivalent to the large jump. The conductance of the fully closed state in such channels is generally less than 2% of the open-state conductance, too low to be differentiated from membrane conductance in the absence of an EIM channel. This ratio is reflected in the current-voltage characteristics of membranes containing large numbers of channels, where ratios of maximum conductance to minimum conductance may range from 50 to 200.EIM channels in oxidized cholesterol bilayers are exceptional in showing only two conductance levels normally. The closed state in this system corresponds to a low intermediate state in the general channel.The conductance of each state apears to be linearly related to the activity of the electrolyte.