It was 1982, I worked in Statens Seruminstitut in Denmark on the susceptibility of P. falciparum to antifolates. I heard  that at David Warhurst's lab they used an isotope method instead of extremely laborious microscopy; wrote to David, if I could visit his lab to learn the method. The answer came immediately "Of course".  I spent 2-3 days in David's lab alongside his Ph D students and learnt what I came for. In addition, I got to know an extraordinarily kind helpful and humorous professor. 

Allan Schapira