This study examines the degree to which young children are influenced by the familiarity of an adult participant in their pretend play with toys which vary in resemblance to highly prototypical objects (e.g., cup-like cups or doll-like dolls). A group of 29 children, mean age 21 months, was divided into two experimental groups balanced by sex; 15 children were assigned to an unfamiliar adult experimenter, and 14 to a familiar one. Each child was observed in two 12-minute play episodes in which pretend play suggestions were made by the experimenter. Results suggest that: (1) pretending with highly prototypical objects is enhanced by thematic proposals of a familiar adult, whereas pretending with less prototypical objects is reduced by these proposals; (2) suggestions from an unfamiliar adult produce a low level of pretending regardless of toy type; (3) children's difficulty in transforming less prototypical objects to suit proposed themes is cognitive rather than motivational; and (4) in the situations studied, girls are more sensitive than boys to the characteristics of people, and boys are more sensitive than girls to the characteristics of materials. (Author/ED)