Structural forms of T cell costimulatory polypeptides are described. These forms comprise an alternative structural domain (i.e., a structural domain having an amino acid sequence which differs from a known amino acid sequence) or have a structural domain deleted or added. The structural forms correspond to naturally-occurring alternatively spliced forms of T cell costimulatory polypeptides or variants thereof which can be produced by standard recombinant DNA techniques. In one embodiment, the T cell costimulatory polypeptide of the invention contains an alternative cytoplasmic domain. In another embodiment, the T cell costimulatory polypeptide of the invention contains an alternative signal peptide domain or has an immunoglobulin variable region-like domain deleted. The alternative structural forms of T cell costimulatory polypeptides can be used to identify agents which stimulate the expression of alternative forms of costimulatory polypeptides and to identify components of the signal transduction pathway which results in costimulation of T cells.