A wild-type Ni-sensitive (Ni(s)) strain of Nostoc muscorum ISU spontaneously yielded mutants resistant to inhibition by 40 μM Ni with a frequency of about 10(-7). A Ni-resistant (Ni(r)) mutant was deficient in the activities of urease and uptake hydrogenase. Cellular Ni uptake in the Ni(s) strain was dependent on concentration (40 to 120 μM) and time (0 to 30 min) (Vmax=0.51 nmol/μg protein.min; Km=92 μM). The Ni bioconcentration factor for such cells ranged between 0.95×10(3) and 1.89×10(3). Ni uptake in spheroplast preparations from Ni(s) cells followed almost the same trend as intact cells except that the bioconcentration factor was slightly less [(0.82 to 1.39)×10(3)]. In contrast, Ni uptake in the Ni(r) intact cells was not concentration dependent and also the uptake was saturated, even at 40 μM, within 10 min. Spheroplasts from the Ni(r) strain showed a Ni bioconcentration factor of 1.19×10(3) compared with 4.41×10(3) for intact cells. The invariably lower Ni uptake by spheroplasts was attributed to altered membrane transport properties.