The uptake of 14C-proline by lung slices and its incorporation into trichloroacetic-acid (TCA)-insoluble proteins and hydroxylation in collagenous proteins were studied. They were found to be linearly increased with the incubation time for as long as 8 h. The effects of different concentrations of bleomycin on these parameters were subsequently investigated. Bleomycin at 10(-3) M significantly reduced the uptake of 14C-proline to 75% of the control value. The incorporation of 14C-proline into TCA-insoluble proteins was reduced to 84%, 78% and 75% of the control at 2.5 X 10(-4) M, 5 X 10(-4) M and 10(-3) M concentrations of bleomycin, respectively. Bleomycin failed to alter the proline pool size of the lung slices at any of these concentrations. In addition, bleomycin also inhibited the hydroxylation of 14C-proline into collagen to 72% and 69% of the control at 5 X 10(-4) M and 10(-3) M, respectively. These findings show that bleomycin-induced increases in collagen synthesis in vivo is probably secondary to inflammatory reactions of the lung and bleomycin per se has little effect on increased collagen synthesis responsible for lung fibrosis.