Analysis of K transport mutants indicates the existence of four separate K uptake systems in Escherichia coli K-12. A high affinity system called Kdp has a Km of 2 muM, and Vmax at 37 degrees C of 150 mumol/g min. This system is repressed by growth in high concentrations of K. Two constitutive systems, TrkA and TrkD, have Km's of 1.5 and 0.5 mM and Vmax's of 550 and 40 at 37 and 30 degrees C, respectively. Mutants lacking all three of these saturable systems take up K slowly by a process, called TrkF, whose rate of transport is linearly dependent on K concentration up to 105 mM. On the whole, each of these systems appears to function as an independent path for K uptake since the kinetics of uptake when two are present is the sum of each operating alone. This is not true for strains having both the TrkD and Kdp systems, where presence of the latter results in K uptake which saturates at a K concentration well below 0.1 mM. This result indicates some interaction between these systems so that uptake now has the affinity characteristic of the Kdp system. All transport systems are able to extrude Na during K uptake. The measurements of cell Na suggest that growing cells of E. coli have very low concentrations of Na, considerably lower than indicated by earlier studies.