The partial resistance (slow rusting) effect and the cultivar effect on latent period (LP) in the barley-leaf rust relationship were highly correlated. Because of this the LP, which can be measured accurately, was used to investigate the nature of this relationship in more detail. The LP''s of 5 isolates were measured on the primary and flag leaves of 6 barley cultivars. Three types of interaction between cultivar and isolate effects on the LP were observed. The isolate effect on LP could depend on the stage of crop development. On the flag leaves, isolates 11-1 and 18 gave longer LP''s than the other 3 isolates. In the seedling stage, isolate 11-1 resulted in a LP shorter than, and isolate 18 in a LP equal to, those caused by the other 3 isolates. The deviation in LP of isolates 11-1 and 18 was far more pronounced in the partially resistant than in the susceptible cultivars. Differential interactions also occurred. Compared with isolates 1-2, 12 and 15, isolate 18 resulted in a longer LP on all cultivars (except ''Julia'') in the flag leaf stage. Since there are no signs that slow rusting or partial resistance to leaf rust in barley is unstable, the results suggest that either partial resistance comprises elements of vertical, race-specific resistance, or the tests used to differentiate horizontal and vertical resistance are unsatisfactory.