Measurement of leading Λ’s and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>Δ</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>’s in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:mo>+</mml:mo><mml:mi mathvariant="normal">Pb</mml:mi></mml:math>Collisions at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mn>19</mml:mn><mml:mi/><mml:mi mathvariant="normal">GeV</mml:mi><mml:mo>/</mml:mo><mml:mi>c</mml:mi></mml:math>

We present the first comprehensive measurement of leading \ensuremath{\Lambda}'s and ${\ensuremath{\Delta}}^{++}$'s in $p+\mathrm{Pb}$ collisions at $19\mathrm{GeV}/c$ using the E941/E864 spectrometer at the AGS. In comparison with the measurement of leading protons and neutrons using the same spectrometer, it is found that the cross section for baryon flavor change is large and strongly depends on rapidity, which is very different from the expectation of simple diquark-quark fragmentation of the incident proton. A suppression of leading \ensuremath{\Lambda} production in the forward rapidity region compared with nonstrange leading baryons is also observed. The relative probability of the projectile proton fragmentation into a ${\ensuremath{\Delta}}^{++}$ versus into a neutron is found to be about 35% in the region of $2.7&lt;~y&lt;~3.1.$ We will discuss the impact of these results on the dynamics of baryon fragmentation and baryon number transport in nuclear collisions.