A cytochrome P-450 (P-450) was purified from abdominal microsomes of untreated and phenobarbital treated susceptible (S+) and insecticide-resistant (LPR) houseflies using HPLC purification procedures. The purified P-450s exhibited the same apparent molecular masses (54,400 dalton) and could not be distinguished from each other on the basis of HPLC chromatographic properties, reduced CO-difference absorbance maxima (447 +/- 0.5 nm), or partial NH2-terminal sequences (MLLLLLLIVVTTLYIFAKL). Since these P-450s were indistinguishable and were immunologically identical to cytochrome P-450lpr we conclude that these P-450s are in fact cytochrome P-450lpr. The level of P-450lpr was increased twofold after PB exposure in the S+ strain, while the level of P-450lpr did not change in the LPR strain. Immunoinhibition of ECOD activity with anti-P-450lpr antiserum suggests that PB treatment in S+ house flies induced P-450lpr, and other forms of cytochromes P-450 with high activity toward this substrate.