Erythrocyte protoporphyrin was measured in calves by the free erythrocyte porphyrin (FEP) test. The FEP of 15 healthy calves was 142.8 +/- 32.4 micrograms/dl (mean +/- SD) of packed erythrocytes or 45.6 +/- 10.3 micrograms/dl of whole blood. Biweekly oral lead administration to 3 calves produced a continuous increase of FEP to between 2,800 and 6,033 micrograms/dl of packed erythrocytes after 20 weeks. Fluorescence scans of diluted blood were used to demonstrate that erythrocyte protoporphyrin from lead-poisoned calves was present as an approximately 2:1 mixture of zinc protoporphyrin and free protoporphyrin. Blood lead concentration increased in calves after lead was given (orally), but had greater weekly fluctuations than FEP. Mild anemia developed in 2 of 3 animals late in the experiment. Basophilic stippling of erythrocytes occurred in 1 calf.