- It is traditionally believed that acute measles causes marked depression of the cellular immunity, but the mechanisms responsible for this phenomenon are not completely understood. We have studied T-lymphocyte markers and response to mitogens in 10 previously healthy, well nourished children under 2 years of age (range 4-23m), with acute measles (between 1 to 4 days after appearance of rash). Depressed lymphocyte counts (< 3000/mm3) were found in 7 children. Per cent T cells were found to be within normal limits in all (44-71% mean 57% vs 57-67% in control mean 63%). T-helper cells were within normal range as compared to controls in 8/10 children (mean 35% vs 37% in controls), in whom the % suppressor cell (mean 24 vs 25% controls) and the H/S ratio (mean 1.5 vs mean 1.7 in controls) were also normal. In 2 children, H/S ratio was inverted (0.5) with a decrease in % helper cells (20%) and increased in % suppressor cell (38%, 40%). Transformation response to PHA, Con A and PWM was tested in all 10 children. A decrease was found in 4 children (including 1 of those with decreased H/S ratio). Our findings suggest that not all cellular immunity markers are affected in children examined during acute measles. The significance of these findings in understanding the interaction of measles virus with the immune system needs further investigation.